Games

Adonthell
A fantasy RPG game available for Linux, FreeBSD and BeOS. MacOS and Windows support will come soon.
http://adonthell.linuxgames.com

CSPoker
CSPoker is a lightweight poker client/server. Client implementations in Java and Flash. XML-based API. (Java) [GPLv2]
http://code.google.com/p/cspoker/

Daimonin
Free isometric real-time MMORPG. 2d/3d graphics, 3d sound effects, digital ambient music.
http://www.daimonin.com/

Danger from the Deep
Danger from the deep (aka dangerdeep) is a Free / Open Source World War II German submarine simulation. [Linux / Windows] (C++)
http://dangerdeep.sourceforge.net

Deliantra
Deliantra is a free software, multiplayer RPG and adventure game, much similar in style to nethack and moria. It is fully graphical and runs in realtime and set in a medieval environment. (GPL) [GNU/Linux,Mac OS X, Windows]
http://www.deliantra.net/

Enigma
A puzzle game based on oxyd. Runs on Windows, OS X, and most open source operating systems. (C++) [GPL]
http://www.nongnu.org/enigma/

Extreme Tux Racer
An OpenGL racing game featuring Tux, the Linux mascot. The goal of the game is to slide down a snow- and ice-covered mountain as quickly as possible.(C++,TCL) [Linux,MacOS X]
http://www.extremetuxracer.com

Fate
A massive multiplayer space trading and combat game for Linux. [GPL]
http://fate.sourceforge.net/

Final Frontier Trader
2D single player space strategy, combat, and trading game. (C++) [OS Independent]
http://fftrader.sourceforge.net

Flight Gear Flight Simulator
The FlightGear flight simulator project is an open-source, multi-platform, cooperative flight simulator development project. Source code for the entire project is available. (OpenGL)[OS Independent]
http://www.flightgear.org/

Free Gamer
Open source games weblog and list.
http://freegamer.blogspot.com/

FreeDroid
Clone of the classic game "Paradroid". (C, Python) [MacOS, Windows 95/98/2000, Linux]
http://freedroid.sourceforge.net/

Frets on Fire
The player has to strike the correct colored squares at the appropriate time during the songs to produce sounds. The objective of this game inspired by Guitar Hero is to finish a song and score the biggest amount of points possible. (Python) [Cross-platform]
http://fretsonfire.sourceforge.net/

The Genius
Chess engine including artificial intelligence, user interface and several features.
http://thegenius.sourceforge.net

Groundfire
A 2D real-time multiplayer tank battling game. (C++) [Windows 9X/2000/XP, Linux]
http://www.groundfire.net/

GtkPool
8-ball pool for Linux with customizable physics.
http://gtkpool.seul.org/

Hawk Software
Developer of the open source HAWK game engine, and the OpenNL portable networking library for Windows.
http://www.hawksoft.com/

Holtz - Zertz and Dvonn board games
Holtz is a GPL implementation of the abstract strategy board games Zèrtz and Dvonn from the GIPF project. (C++) [Linux, Windows] [GPLv2]
http://holtz.sourceforge.net

Hot Potato Online
This game is a fast paced arena sport where players try to explode the opposition using a short-fused potato bomb.
http://hponline.sourceforge.net

irrlamb
irrlamb is a 3D game that probably involves a lot of physics and frustrating gameplay. (C++) [Linux, Windows] [GPL2]
http://code.google.com/p/irrlamb/

jQuizShow
Free quiz game based on the "Who Wants to Be a Millionaire" game show. Add your own questions for classroom use or for fun. [Java, OS Independent, GPL]
http://quizshow.sourceforge.net

Kraptor
2D shoot'em up made with Allegro. (C) [GNU/Linux, Windows, DOS]
http://kraptor.sourceforge.net/

Libregamewiki
The encyclopedia of free (as in freedom) games with free content.
http://libregamewiki.org/

Linux Game Development Center
The LGDC provides concise and accurate articles about Linux gaming news, development tutorials, and reviews of the latest tools and resources.
http://lgdc.sunsite.dk

MUSoSu
A free software project for playing, solving, and generating Sudoku games. (Java) [OS Independent]
http://musosu.sourceforge.net/

Open Game Source
A web magazine devoted to open source game development.
http://www.identicalsoftware.com/ogs/

OpenLieroX
OpenLieroX is the open source clone of the famous Liero (the realtime Worms) game.
http://openlierox.sourceforge.net

Planet SolarWolf
An updated clone of SolarFox which was released in 1983 for Atari 2600. The objective is to get through 60 levels collecting space boxes.
http://www.pygame.org/shredwheat/solarwolf/

Playing the Open Source Game
Shawn Hargreaves has several Open Source projects and interesting articles about the future of Open Source games.
http://www.talula.demon.co.uk

SecurePlay Open Source Development SDKs
Build games on Secureplay sourcecode libraries in Flash/Java C++. Create secure game transactions for multiplayer games (player management, state/event control, random events, secret events) Platform independent Encrypted pluggable communications.
http://www.secureplay.com/products/products.htm

Small Apps and Games
Small apps and games developments.
http://www.freewebs.com/elfofthefire/index.htm

Snip
Open-source Tetris implementation.
http://snip.sourceforge.net

Spice Trade
Colonize Europe in this Java based, open source game. Live an adventure in the 12th century world.
http://www.spicetrade.org/