Cube Engine
Cube is an open source multiplayer and singleplayer first person shooter game built on an entirely new and very unconventional engine. Cube is a landscape-style engine that pretends to be an indoor FPS engine, which combines very high precision dynamic occlusion culling with a form of geometric mipmapping on the whole world for dynamic LOD for configurable fps & graphic detail on most machines. Uses OpenGL & SDL.
Assault Cube
AssaultCube is a FREE, multiplayer, first-person shooter game, based on the CUBE engine.
Taking place in realistic environments, with fast, arcade gameplay, it's addictive and fun!
With efficient bandwidth usage, it's low-latency and can even run over a 56Kbps connection.
It's tiny too, weighing in at a lightweight 40 MB package available for Windows, Mac and Linux.
On the correct settings, it can even run on old hardware (Pentium III and above).
Cube 2: Sauerbraten
Cube 2: Sauerbraten is a free multiplayer & singleplayer first person shooter, the successor of the Cube FPS.
Much like the original Cube, the aim of this game is fun, old school deathmatch gameplay and also to allow map/geometry editing to be done cooperatively in-game.
Red Eclipse
Red Eclipse is a fun-filled new take on the casual first person arena shooter, built as a total conversion of Cube Engine 2, which lends itself toward a balanced gameplay, with a general theme of agility in a variety of environments.
Tesseract
Tesseract is a first-person shooter game focused on instagib deathmatch and capture-the-flag gameplay as well as cooperative in-game map editing.
Tesseract provides a unique open-source engine derived from Cube 2: Sauerbraten technology but with upgraded modern rendering techniques. The new rendering features include fully dynamic omnidirectional shadows, global illumination, HDR lighting, deferred shading, morphological/temporal/multisample anti-aliasing, and much more.
Revelade Revolution
Revelade Revolution is an open-source First Person Shooter (FPS).
The game features fast-paced gameplay in multiplayer modes.
Players are faced against swarms of zombies who have recently overtaken the city and are slowly spreading across the world.