The Cube Engine is a 3D renderer, created by Wouter van Oortmerssen.
It is written in C++, and its primary focus is on rendering outdoor scenes, but it can display simple indoor areas as well. It features in-game map editing without the need of any map precompilation. It also features cooperative map editing with other players. It is under the zlib/libpng license. It is used by the games Cube and AssaultCube which are mostly free software, although some small amounts of multimedia is from proprietary sources. Thus far no one has made entirely free versions.
Sauerbraten (engine) - The successor to the Cube Engine.