Feel free to suggest any free games that you feel are fun and polished enough that others would enjoy them. A short description, some basic info, and a 400x300 image would be great. Just make a post in this topic and I'll add it to the list.
Note that this is not for online games! Also, shareware games that are not feature-crippled are allowed on the list.
Key:
100% Free OSS = Completely free, source code available.
100% Free = Completely free.
Shareware = Free version is essentially as good as the full version and is not crippled.
Platforms: The systems/OSes the game will run on; Windows, Linux, Mac, and Various Consoles are possibilities, but note that some Windows games may run in WINE/Parallels.
Multiplayer = Support for extra players; Internet, LAN for network play. For splitscreen games, use Splitscreen(#). For turn-based or real-time hotseat games that don't split the screen, use Hotseat(#).
Difficulty = The difficulty of the game; Easy, Medium, or Hard.
Longevity = The time you'll probably spend on the game; Fair, Moderate, or High
The list! (An ongoing WIP)
Action - 3D Shooters
Black Shades
http://wolfire.com/blackshades.html
100% Free
Platforms: Windows, Linux, Mac
Multiplayer: None
Difficulty: Medium
Longevity: Fair
You're a psychic bodyguard whose goal is to protect the VIP by any means necessary from various kind of assassins and zombies. Not only do you get a variety of weapons to protect the VIP, but you also get some useful psychic powers, such as bullet-time.
Action - 2D Shooters
Grand Theft Auto 2 (1)
http://www.rockstargames.com/classics/
http://en.wikipedia.org/wiki/Grand_Theft_Auto_2
100% Free
Platforms: Windows
Multiplayer: Internet, LAN
Difficulty: Medium
Longevity: High
Everybody here knows about GTA, but here's (two) of the classic versions, available for free from Rockstar. Drive around, steal cars, and go wild.
Gusanos
http://gusanos.sourceforge.net/
http://en.wikipedia.org/wiki/Gusanos
100% Free OSS
Platforms: Windows, Linux
Multiplayer: Internet, LAN, Splitscreen(2)
Difficulty: Easy
Longevity: Fair
Gusanos is sort of a mix between Worms and Soldat. It's also somewhat of a clone to the games Liero and LieroX. The ninja rope that every player has adds to the frenzied gameplay, and it all comes together for a fun, fast-paced game. It's also very moddable and has LUA support, for those who like to tinker. Numerous polished mods can be found on the official forums, and I would recommend either Blasted or DOOM. It doesn't have much of a community.
Soldat
http://www.soldat.pl/
http://en.wikipedia.org/wiki/Soldat
Shareware
Platforms: Windows
Multiplayer: Internet, LAN
Difficulty: Easy
Longevity: Moderate
I'm sure most people here have played this, but Soldat is a 2d shooter with many gamemodes and a large community. It's fairly moddable.
Wings 2
http://www.wings2.net/
Shareware
Platforms: Windows, Linux
Multiplayer: Internet, LAN, Splitscreen(4)
Difficulty: Easy
Longevity: Fair
Wings 2 belongs to a mainly-Finnish genre of games called "luolalentely", or "cave-flying". Basically you fight gravity while flying through tight corridors, shooting your enemies. Unfortunately it has nearly no community, but it's a blast in splitscreen.
Adventure - Sidescrolling
Cave Story
http://agtp.romhack.net/project.php?id=cavestory
http://en.wikipedia.org/wiki/Cave_Story
100% Free
Platforms: Windows, Mac
Multiplayer: None
Difficulty: Medium
Longevity: High
Cave Story is a very polished sidescrolling adventure. Its story is great, and if you haven't played it yet, you owe it to yourself to.
Ben Croshaw's Point-and-Click Games (1213, Trilby's Notes, 7 Days a Skeptic, 5 Days a Stranger)
http://www.fullyramblomatic.com/games.htm
100% Free
Platforms: Windows
Multiplayer: None
Difficulty: Medium
Longevity: Moderate
"1213 is an excellent science-fiction platformer, split into 3 episodes, that plays out a bit like the original Prince of Persia. The gameplay itself is rather simple and the only real challenge are the boss fights. You platform around the facility, shoot up some baddies, and solve puzzles."
"5 Days a Stranger, 7 Days a Skeptic, and Trilby's Notes are a trilogy of horror point-and-click adventure games with excellent writing and characters. If you like point-and-click adventure and horror games then these games should please you. I liked 5 Days a Stranger quite a bit more than 7 Days a Skeptic, but that's not to say 7 Days is bad."
Arcade
Super Mario War
http://smw.72dpiarmy.com/
100% Free OSS
Platforms: Windows, Linux, Mac, Various Consoles
Multiplayer: Hotseat(4)
Difficulty: Easy
Longevity: Fair
In Super Mario War there are a variety of gamemodes, but most revolve around stomping your enemies and killing them. It's fast-paced, mindless fun, and it's surprisingly addictive. Internet multiplayer is in the works.
Fighting
Little Fighter 2
http://lf2.net/
100% Free
Platforms: Windows
Multiplayer: Internet, LAN, Hotseat(4)
Difficulty: Easy
Longevity: Fair
Little Fighter 2 is a fighting game with multiple game modes. One plays like Streets of Rage or Final Fight. The game throws generic enemies your way and it's your job to beat the everloving snot out of each and every single one of them. There's also a standard fighter mode where you fight characters that are just as powerful as you are. Each character is fairly different and has multiple special attacks.
Horror
Penumbra
http://frictionalgames.com/penumbra/
100% Free
Platforms: Windows
Multiplayer: None
Difficulty: Easy
Longevity: Fair
"Penumbra is not your standard survival horror game. It's all about puzzle solving since you don't actually get a gun. That's not to say there aren't monsters though. The game has some pretty kickin' graphics for a freeware game which certainly adds to the horror."
Puzzle - 3D
Narbacular Drop
http://www.nuclearmonkeysoftware.com/
http://en.wikipedia.org/wiki/Narbacular_Drop
100% Free
Platforms: Windows
Multiplayer: None
Difficulty: Medium
Longevity: Fair
Narbacular Drop is similar to Valve's upcoming Portal. You use a single pair of portals to solve puzzles and escape rooms. When objects fall into one portal, their velocity is carried through to their exit of the other portal. This allows for some interesting solutions to the puzzles in the game. It has custom level support.
Neverball
http://icculus.org/neverball/
http://en.wikipedia.org/wiki/Neverball
100% Free OSS
Platforms: Windows, Linux, Mac
Multiplayer: None
Difficulty: Hard
Longevity: Moderate
Neverball is a marble maze game very similar to Super Monkey Ball. You tilt the game world, rolling your ball around and collecting coins and then getting to the goal. But beware, it can be as frustrating as it is addictive. There are 3 levelsets that come with it ranging from easy to impossible (I know, I've tried...
), and more are available on their forums.
Simulation
Rigs of Rods
http://rigsofrods.blogspot.com/
http://en.wikipedia.org/wiki/Rigs_of_Rods
100% Free
Platforms: Windows, Linux
Multiplayer: None
Difficulty: Medium
Longevity: Fair
Here's one for all the physics games fans out there. Rigs of Rods is a truck driving simulator, but it's actually rather fun. It also has cars and planes. The vehicles are made from rods instead of being solid bodies, and this allows them to deform when colliding with other objects. Multiplayer is in the works. Be advised that this requires a rather powerful computer to run smoothly.
Strategy - "God"/Tycoon
OpenTTD
http://www.openttd.org/
http://en.wikipedia.org/wiki/OpenTTD
100% Free OSS
Platforms: Windows, Linux, Mac
Multiplayer: Internet, LAN
Difficulty: Easy
Longevity: High
OpenTTD "simulates" the management of a transportation company. You move cargo from place to place via different sets of vehicles, fill subsidies, and try to drive your competitors out of business. Highly addicting once you get into it. Note that it still uses the graphics files from the original Transport Tycoon Deluxe, which means they are unable to package them with the game. But you can still find OpenTTD and the data packaged together on torrent sites.
Strategy - War
Spring
http://spring.clan-sy.com/
http://en.wikipedia.org/wiki/Spring_(computer_game)
100% Free OSS
Platforms: Windows, Linux(WIP), Mac(WIP)
Multiplayer: Internet, LAN
Difficulty: Hard
Longevity: High
Spring is a heavily-moddable real-time strategy game in the style of Total Annihilation or Supreme Commander. Thus, instead of being like Command and Conquer, its learning curve is a bit more steep. Multiplayer games can take a while to complete, but there is a lot more strategy for a cunning player to tap into. Its wide selection of mods allows for very varied gameplay.
Warzone 2100
http://wz2100.net/
http://en.wikipedia.org/wiki/Warzone_2100
100% Free OSS
Platforms: Windows, Linux, Mac
Multiplayer: Internet, LAN
Difficulty: Medium
Longevity: Moderate
"While I'm not a huge fan of RTS games, I like Warzone 2100. At the beginning of the campaign mode, you start with hardly anything besides contruction units and basic tanks. As you move throughout the map, you find artifacts that you can research that allow you to build different kinds of units. And I do mean 'build', because you construct your units yourself by connecting different parts."
