Sauerbraten (also known as Cube 2 or Sauer) is a cross-platform, Quake-like, first-person shooter.
The game started started as a redesign to the original Cube Engine and shares most of its design goals and philosophy with its predecessor, but using a new 6-directional heightfield (or octree) world model. This allows much more complex level geometry and easier editing.
The aim of the Sauerbraten open-source project is not necessarily to produce the most features and eye candy possible, but rather to allow map/geometry editing to be done in real-time within the game environment, while keeping the source code small and elegant.
Grab the free full CTP version of Sauerbraten for Windows from GP Downloads and try it for yourself!