Sauerbraten (Cube 2) Portable for Windows

Sauerbraten (Cube 2) Portable is a free multiplayer or singleplayer 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 eyecandy possible, but rather to allow map or geometry editing to be done dynamically in-game, to create fun gameplay and an elegant engine.

The engine supporting the game is entirely original in code and design, and its code is Open Source (ZLIB license, read the docs for more on how you can use the engine).