Cube 2: Sauerbraten

This is a completely free FPS game for GNU/Linux, Windows, or Mac machines. It features a relatively intuitive map editor, loads of scenarios and playing modes, and plenty of documentation to get you started. From their website: "Cube 2: Sauerbraten is a free multiplayer/singleplayer first person shooter, built as a major redesign of the Cube FPS. Much like the original Cube, the aim of this game is not necessarily to produce the most features & eyecandy possible, but rather to allow map/geometry editing to be done dynamically in-game, to create fun gameplay and an elegant engine." Source: The Game Website.