The original Cube is also fun, at least as a singleplayer game and comes with a lot of SP maps. The SPMP (or MPSP, i don’t remember :-P) mode also allows playing the MP maps with randomly spawned bots.
The AssaultCube fork adds more realistic environments (relatively to the original game anyway), though it is fully MP oriented. But you can still play with bots.
Cube 2 Sauerbraten is pretty good! Free arena shooter with built-in level editor
The original Cube is also fun, at least as a singleplayer game and comes with a lot of SP maps. The SPMP (or MPSP, i don’t remember :-P) mode also allows playing the MP maps with randomly spawned bots.
The AssaultCube fork adds more realistic environments (relatively to the original game anyway), though it is fully MP oriented. But you can still play with bots.