Doom was the classic 3D shoot'em'up game from id Software. PrBoom is a game engine for playing Doom levels, which runs on both Windows and Linux/Unix systems. It uses the SDL graphics library. PrBoom a very faithful Doom port, with great effort taken to be compatible with as many original Doom levels and demos as possible.
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.
The Interreality project is a collection of Open Source, Free Software projects based on the Virtual Object System (VOS) platform. The goal is to develop a free and open platform for multi-user 3D virtual reality games, applications, and interactive, collaborative 3D virtual spaces. VOS is a powerful and generic network object framework designed to support a variety of high-performance applications. It is the basis for Ter'Angreal, a general-purpose browser application for entering and participating in 3D worlds.
L3D is a 3D library for developing OpenGL applications under Linux. The library has a main display loop, and call-back functions similar to those in GLUT. It has a graphical console for output of information. The goal of this project is a library which is small and hopefully scalable.