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

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

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

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

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

K2W RPG
A collection of open source client, server, and editor to build online role playing games.
http://www.k2wrpg.org/

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

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

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

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

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

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.
http://sekoya-games.sourceforge.net/

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

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/

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

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

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/

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

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

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

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

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/

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

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

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

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

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

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

Planet Umi
Grow a planet of silly Umis. (python/pygame) (GPL3) [OS X, Windows]
http://planetumi.com