Cube 2: Sauerbraten

Cube 2: Sauerbraten (also known as Sauer, German for sour roast ) is a cross-platform, Quake-like first-person shooter that runs on Microsoft Windows, Linux, FreeBSD, and Mac OS X using OpenGL and SDL. The game features single-player and multiplayer gameplay and contains an in-game level editor, its main feature. The game engine is free and open source software, under the zlib License, with commercial support available from the developer's own business counterpart.