Adonthell - A fantasy RPG game available for Linux, FreeBSD and BeOS. MacOS and Windows support will come soon.
CSPoker - CSPoker is a lightweight poker client/server. Client implementations in Java and Flash. XML-based API. (Java) [GPLv2]
Daimonin - Free isometric real-time MMORPG. 2d/3d graphics, 3d sound effects, digital ambient music.
Danger from the Deep - Danger from the deep (aka dangerdeep) is a Free / Open Source World War II German submarine simulation. [Linux / Windows] (C++)
Enigma - A puzzle game based on oxyd. Runs on Windows, OS X, and most open source operating systems. (C++) [GPL]
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]
Fate - A massive multiplayer space trading and combat game for Linux. [GPL]
Final Frontier Trader - 2D single player space strategy, combat, and trading game. (C++) [OS Independent]
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]
FreeDroid - Clone of the classic game "Paradroid". (C, Python) [MacOS, Windows 95/98/2000, Linux]
The Genius - Chess engine including artificial intelligence, user interface and several features.
Groundfire - A 2D real-time multiplayer tank battling game. (C++) [Windows 9X/2000/XP, Linux]
GtkPool - 8-ball pool for Linux with customizable physics.
Hawk Software - Developer of the open source HAWK game engine, and the OpenNL portable networking library for Windows.
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]
Hot Potato Online - This game is a fast paced arena sport where players try to explode the opposition using a short-fused potato bomb.
irrlamb - irrlamb is a 3D game that probably involves a lot of physics and frustrating gameplay. (C++) [Linux, Windows] [GPL2]
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]
K2W RPG - A collection of open source client, server, and editor to build online role playing games.
Libregamewiki - The encyclopedia of free (as in freedom) games with free content.
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.
MUSoSu - A free software project for playing, solving, and generating Sudoku games. (Java) [OS Independent]
Open Game Source - A web magazine devoted to open source game development.
OpenLieroX - OpenLieroX is the open source clone of the famous Liero (the realtime Worms) game.
Playing the Open Source Game - Shawn Hargreaves has several Open Source projects and interesting articles about the future of Open Source games.
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.
Sekoya - open source mmorpg - Sekoya is a gathering of students trying to develop nice open source games. Our main project is a MMORPG running under Linux and Windows.