Tremulous is a computer game that blends a team-based first-person shooter with elements of realtime strategy. The objective of the game is to annihilate the opposing team. This is achieved by eliminating the opposing players and removing their ability to respawn by destroying their spawn structures.
Sauerbraten (a.k.a. Cube 2) is a free multiplayer/single player first person shooter, built as a major redesign of the Cube FPS. Much like the original Cube, the aim of this game is not necessarily to produce the most features and eye candy possible, but rather to allow map/geometry editing to be done dynamically in-game, to create fun gameplay and an elegant engine.
Py-Lepton is a high-performance, pluggable particle engine and API for Python. The engine is designed to be very flexible and does not rely on any other libraries directly. You can use it either with OpenGL (via pyglet or PyOpenGL), or with pygame by selecting the appropriate renderer. Examples are provided using pyglet and pygame. Note that this is in an alpha state, so expect the API to change somewhat in future releases.
BZFlag is a 3D, multiplayer, tank battle zone, capture the flag game that pits players against each other in a networked environment. It runs on Windows 95/98/NT/2000, Linux, MacOS 10.x, Irix, Solaris, and others. An OpenGL accelerator is highly recommended, but it is playable with 3D in software.
Nexuiz is a fast-paced, chaotic, and intense multiplayer first-person shooter game, focused on providing basic, old style deathmatch. It is extremely modder-friendly. Nexuiz is built on the power of the Darkplaces engine, which is a heavily modified version of the original Quake. Darkplaces features realtime lighting and stencil shadows, bumpmapping, gloss, bloom, and totally rewritten network code that supports up to 64 players on a single server.
Alpha Shooter is a 3D first-person shooter game with a sci-fi setting. It features multi-platform support (Linux, Mac OS X, CygWin, and Windows) and, since version 0.1.0, its own reusable game engine (Neutron). The game and its engine will evolve over time to incorporate simulated physics and AI, networking, audio, scripting, and more.