Cube 2: Sauerbraten
Cube 2: Sauerbraten is a Free single and multi player 1st person shooter game with some satisfying fast oldskool gameplay. A large variety of gameplay modes from classic SP to fast 1 on 1 MP and objective based teamplay, with a great variety of original maps to play on.
Level editing has never been so much fun: a press of a key allows you to modify the geometry / textures / entities in-game, on the fly. Even more novel, you can make maps together with others online, in the unique "coop edit" mode (!)