Cube 2: Sauerbraten

Cube 2: Sauerbraten (German for "sour roast", also known as Sauer) 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, Dot3 Labs.

The game media is released under various non-free licenses. The aim of the project is not to produce the most features and highest-quality graphics possible, but rather to allow map-editing to be done in real-time within the game, while keeping the engine source code small and elegant.