Sauerbraten:
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.
The engine supporting the game is entirely original in code & design, and its code is Open Source (ZLIB license, read the docs for more on how you can use the engine).
dot3 labs is a company started by the creators of Sauerbraten that offers commercial support.
Authors, feature list and more: see the reference documentation or the wiki.
I usually play insta ctf, you have to capture the opposite team's flag and bring it to your base (If the other team stole your flag you cant make a point). Since its INSTA ctf you die from 1 shot. I play with the name "Tomy" and I recommend that game.
All the things you need to know, are here. Oh and the maps are basically squares but ultra edited with tons of kewl thingys. But you can make your own map. I suck at that but Im learning =P
http://cube.wikispaces.com/