If you're anything like me, you have probably spent hours (or even days) of your life looking for free games on the net. Yes, there's no shame in admitting it, getting things for free is fun. And playing games is fun. So playing free games is... well you do the math.
The only problem is that in the vast jungle we have come to call 'the Internet', you'll come across dozens of abandoned, semi-free or just plain crappy games before you can find one that actually makes it worth the while. Being the kind person that I am, I decided that I would share my experience in the field, and give you a list of games worth checking out.
I've only included open source games that don't charge money for any feature, because these should match any definition of free you might use. Besides, free closed source games usually either require proprietary data files or don't stay free for very long.
I don't claim this list to be complete, and if there is a game that, you think, absolutely needs to be included in this overview, feel free to let me know.
I've tagged 
my personal favourites.
Space simulation
One of my favorite genres, space sims have been around since the dawn of gaming. While remaining fairly simple, they can give the player the impression of a seemingly endless universe with vast planets and impressive space ships.
Oolite
Runs on Mac OS X, Windows and Linux
Oolite is a clone of the classic Elite, which was released on virtually every early home computer platform. Elite is the game most modern day space sims are based on, and it's trading system is mimicked by an endless list of other games.
If you like retro space sim games, it is your sworn and sacred duty to check this one out. Moreover, it's an ideal way for those Elite veterans out that to relive their past.
Allegiance
Only runs on Windows
Some would argue that this one is not really open source, but I feel it is. This game was developed by Microsoft Research and released under their Shared Source license. The game is now maintained by volunteers, so that's open enough for me to include it here.
Allegiance is an online space sim with several factions for the user to choose from, each with their own spacecrafts and properties. The player can pick a role, from turret gunner to commander. Emphasis is put on squadron cooperation and strategy. Help your faction evolve and emerge victorious.
Vegastrike
Runs on Mac OS X, Windows and Linux
Vegastrike is a completely 3D space sim, with some impressive graphics. Flying looks pretty dated, so let's hope they have a big update planned sometime soon.
There are many mods for it though, both single player and multi player. (UPDATE: They have released a new version and seem to be working hard on improving the game these days!)
First Person Shooters
Dominating the gaming industry since Wolfenstein 3D, first person shooters are very popular among gamers all over the world. Arguably involving more skill than any other gaming genre out there, FPS games are in high demand. Here are a few excellent open source shooters:
Nexuiz
Runs on Mac OS X, Windows and Linux
Nexuiz is a good old shoot-em-up arena FPS, quite similar to Unreal Tournament. Featuring superb graphics, simple gameplay and a large arsenal of weapons, this game is perfect if you just want to shoot everything that moves. It also has many of the classic team play game modes, such as capture the flag and team deathmatch, which makes it a nice game to play with a couple of friends over the LAN or the internet.
One of the few complete, featureful open source shooters out there. A definite must-try.
Sauerbraten
Runs on Mac OS X, Windows and Linux
Sauerbraten is the successor of the popular Cube game. Like it's older sibling, this is more of a proof-of-concept of the revolutionary in-game map editor and a showcase for the impressive engine than a real game. It only features a single model, but it has many different maps and a number of different game modes.
Although it's not really meant to be a full game, the engine is absolutely amazing. It automatically scales down on older, slower computers, but it can full advantage of the powerful video card in your hot rod gaming rig. Although none of my computers allow me to see it, I hear the water is especially pretty.
Open Arena
Runs on Mac OS X, Windows and Linux
Mainly a content package for the open sourced Quake III engine, this game aims to be an open source version of the popular Quake III: Team Arena game. I haven't tried it out myself, but it seems to be quite popular and is still under active development.
PrBoom
Runs on Mac OS X, Windows and Linux
An up-to-date clone of the legendary Doom game. Features OpenGL graphics. 'nuf said.
Aleph One
Runs on Mac OS X, Windows and Linux
Another clone of one of the classics, this one is an incredibly enhanced version of Marathon. With high res graphics, many extra scenarios and an interesting online multiplayer feature, this game is a must-have for anyone who wants to respect the gamer within him- or herself.
Real Time Strategy
Want to do something more than shoot other peoples brains out or collecting experience points? Want to use your brain and outsmart enemy forces? Or do you just want to build a huge army and right-click your opponent's base? In either case, here are some RTS games for you to enjoy:
netPanzer
Runs on Windows and Linux, Mac OS X version is outdated
This is one of my personal favorites. Simple, multiplayer strategy fun. Capture a base to build units, and try to expand your network. With no resources, every base can produce units all the time, with small tanks building faster than big, bad rolling fortresses. Capture as many bases as you can to win, and team up with other players to overpower a dominant commander.
TA Spring
Runs on Windows and Linux, Mac OS X version is in pre-release status
Spring is a 3D RTS with impressive graphics. It is set in Total Annihilation universe, and aims to surpass the gameplay of the original TA games. If you're a fan of Total Annihilation, you should check it out.
Glest
Runs on Windows and Linux, Mac OS X status is not clear
Glest is a beautiful game with amazingly detailed 3D models and environments. It is a mediaeval-fantasy themed game, where mythical beings and human soldiers fight using swords, bows, magic and so much more. There are two factions: the tech faction, which uses mechanical and physical weapons, and magic faction, which uses magical incantations to achieve victory.
Turn based strategy
Personally, I don't like real time strategy half as much as I like turn based strategy. Partly because it gives you time to think, but also because you can get to understand the underlying system a lot better to completely master the game. You can actually develop techniques that have a reliable effect, depending on the ruleset the game uses. I prefer the rule-based strategy pattern over any real time mechanism I encountered so far. Turn based strategy also feels much more retro, and I happen to like that kind of thing. If you do as well, check these out.
UFO: Alien Invasion
Runs on Windows, Linux and Mac OS X (UPDATE! Mac OS X now supports PowerPC as well!)
UFO Alien Invasion is based on the good old X-Com series. Although I never played the original titles, I enjoyed the clones a lot, and this one puts the icing on the cake. Unlike a lot of clones out there, this one doesn't require any proprietary data files, but offers its own artwork. What's more, it's even using 3D graphics, with fancy animations, smooth models and so much more.
Research alien technology, develop new equipment and save the earth from the invading aliens. You know you want to. (I should probably warn you, this game is very addictive)
FreeCiv
Runs on Linux and Windows and Mac OS X (Now runs without X11 thanks to the all new SDL client!)
As most of my fellow Civilization-veterans will confirm, it's best not to get to know this game. That's right. I'd go even further and say it is in your own interest to deny it's very existence. Never, ever, under any circumstances, start playing a Civilization title. You'll loose days, if not weeks of your life, and by the time you eventually stop playing, you might find your social life severely damaged. Like it's source of inspiration, FreeCiv is highly addictive, but fails to reach for the stars of TBS-heaven. The AI is absurdly good, even on the lowest setting I failed to win a single game. (UPDATE: After practicing a bit more and playing some online games, the AI is not all that hard to beat) And I've beaten the original games on the highest difficulty level.
The multiplayer feature tries to make up for this, and partly succeeds. The graphics are comparable with Civ II in terms of quality, and the technology tree is H-U-G-E. If you have a friend that appreciates a decent turn based strategy game, this might just be one of the best things to do with your spare time.
Battle for Wesnoth
Runs on Linux, Mac OS X and Windows
If you like turn based strategy, this is a must try. An engaging story line, intuitive yet powerful interface and allows you to use your tactical wits to exploit predictable friends or AI players. Don't think that the AI is primitive though, the computer controlled armies seem to be very good at exploiting your weaknesses as well. Loading a game and trying over results in completely different moves by the AI.
Wesnoth is also one of the few free strategy that added role-playing elements to the mix and actually got it right. Leveling your soldiers and heroes is highly rewarding, and the game has an interesting way of letting characters improve even after they reached the maximum level.
A very engaging game, with lots of great quality campaigns, as well as some user created content that can be downloaded in-game. The multiplayer is just good fun. Teaming up with human players against computer opponents or just proving your tactical superiority to your friends, this game doesn't disappoint. Not even for a split second.
MMO RPGs
Possibly the most popular format of games these days are MMO RPGs. Most MMO games require a monthly fee to play, often more than a quarter of the average student's income. To fulfill the needs of the poorer gamer out there, there is an alternative:
Planeshift
Runs on Mac OS X, Windows and Linux
Planeshift is a typical, fantasy themed MMO RPG. The player first customizes his character by picking a race, gender, name, religion, and several events that have had an influence on the characters life so far. For instance, you can pick where you grew up, what your parents did for a job, how good they were at it, if something special happened when you were born, and much more.
This game is still a work in progress, some races are missing artwork, and the animations are pretty shocky, the game is set a complete and persistent 3D world, with many different skills for your character to master.
Although incomplete, this game is already quite interesting to play, and has an active gamer community. Unlike many other MMO games, most people here are very friendly towards beginners and are eager to help out.
Arcade
Simple games are often best, and these games use simple concepts to focus on the ultimate gameplay experience. Many are based on classics, but some are more original.
BZFlag
Runs on Windows, Mac OS X and Linux
BZFlag is your good old tankhunter game, with some fancy enhancements. It allows you to play against hundreds of other players around the world in team-based capture the flag games. Tanks can jump and the levels have many warp points that allow you to get to elevated locations. There are also many upgrades for you to pick up, some of which are actually both funny and useful. For instance, there is a powerup that will make your tank as thin as a piece of paper, making it much harder to hit you head on. The game also has nice bling effects to go with your energy-based cannon balls.
Frozen Bubble
Runs on Linux only. (Much) older versions are available for all major platforms
A very popular arcade game, based on Bust-a-Move. Features cute penguins, nice graphics and multiplayer gameplay. Very addictive, best avoided if you want to do something productive in the near future.
Neverball
Runs on Linux, Mac OS X, Windows
A Super Monkey Ball inspired arcade game, Neverball is a simple yet enjoyable game. Instead of controlling the ball, you get to control the level in this game. Use physics to force the ball to collect coins and reach the goal. Best played with a joystick or analog gamepad.
Scorched 3D
Runs on Windows, Linux and Mac OS X
Scorched 3D is a cannon game based on the old DOS game Scorched Earth, but takes the concept into the 21st century. Features a 3D world and beautiful explosions. Pick your unit from the standard set of tanks, or the more original Bender model, an AT-ST Walker, a Protoss Photon Cannon or Terran Siege Tank. The controls are easily mastered. Scorched3D offers hours of fun when playing with friends.
Supertux
Runs on Windows, Linux and Mac OS X
A Super Mario Bros like jump'n'run sidescroller. This game has a limited set of enemies, but plenty of levels to run through, and has beautiful graphics and annoying music. Everything you've come to expect of a modern day sidescroller.
Simulation
According to sales statistics, this is indisputably the king of gaming genres. I've selected some gems in this genre. I'm not a fan of sim games, but these are some exceptions that managed to grab my attention and hold it. For an exuberantly long time.
LinCity NG
Runs on Windows and Linux. Mac OS X binary available for Intel only. (You can compile the PPC version from source)
A SimCity like game, this game has come a long way since the original LinCity project. This next-generation incarnation sports 3D graphics, while remaining loyal to the original isometric viewing point. You wouldn't want to play a City sim without that beloved 45-degree angle, now would you? This game has a rich collection of buildings for you to fill up your town.
OpenTTD
Runs on Windows, Linux and Mac OS X
Those of you who have played the original Transport Tycoon Deluxe probably need no convincing to rush over to the OpenTTD site and grab a download. Those of you who haven't are probably thinking Transport Tycoon is yet another tycoon with an uninspired and boring subject like public transportation. Think again: this game is guaranteed to offer you countless hours of fun. Starting out with a modest budget, you can build bus stops, truck stations, train stations, airports and harbors. Build roads and lay out a network of railroad tracks to make your company the biggest transport corporation out there. Buy your opponents companies or become a shareholder to maximize your profit. Buy top-of-the-line monorails and airplanes. See cities grow because of the influx of people and goods you made possible. This game has a wealth of possibilities for you to enjoy, and combines this with the original retro isometric artwork.
Don't worry about the fact that you need the original game, it's available for download at Abandonia for free. And your not breaking any laws, since it's abandonware.
Conclusion
Well, I hope these can satisfy your gaming needs, at least for a while. I've already been pointed to some other games, and I might add them after I give them a try. And if anyone ever claims open-source game development is never going to do anything worth mentioning, point them here. In my experience, these games often surpass commercial titles in gameplay, storyline and content.
Kudos to Matti and Dominic for proof-reading and corrections.