Browser Based

Blacknova Traders
Multi-player space exploration and strategy game (PHP) [GPL].
http://www.blacknova.net/

BMTron
Multi-player tron light cycles game for 1-4 players with teamplay mode. (Java) [GPL]
http://kryshen.pp.ru/games/bmtron.html

Legend of the Green Dragon
A remake of the classic BBS Door game, Legend of the Red Dragon (aka LoRD) by Seth Able Robinson. It is written in PHP, using a MySQL backend, the game's source code is licensed under the GNU GPL.
http://www.lotgd.net

phpRPG
A multiplayer fantasy roleplaying game driven by PHP and MySQL, with aims to develop a web-based game engine which shares common qualities between multiple user dungeons (MUDs), paper and pen RPGs and computer RPGs. [GPL]
http://phprpg.org

Quantum Star: Generations
A space strategy game involving Mining, Combat, Exploration, Planets and Clans played in real time. (PHP/SQL) [AGPL]
http://www.quantum-star.com/

SpyWar
SpyWar is an online strategic game in development phase. (PHP, PostgreSQL) [Linux] [GPL] {status: halted}
http://spywar.sourceforge.net/