Sauerbraten is the follow up to Cube, a fun freeware shooter released a few years ago. Like Cube, Sauerbraten focuses on letting the user make their own maps in-game. I myself didn’t get to try out the editor, as it seems my system isn’t haas enough to run the game fully. I did, however, enjoy a few levels of mock deathmatch (damn imp…things). The engine is VERY solid, though as a game it’s still lacking a bit (The single player campaign isn’t finished for example, but there are quite a few single player mock deathmatch levels). You can also head over to Quadropolis for even more maps.
But a free FPS isn’t the only game that Sauerbraten (The engine) has up it’s sleeve, there’s an RPG called Eisenstern in development using the engine, and it’s also been licensed for a commercial use by Proper Games, who seem to be pushing the engine to new heights.