Um....
Cube 2: Sauerbraten (Freeware - Mac OS X, Linux, Windows), its pretty fast, we could play some rounds instagib team on venice. It has a co-op edit mode which was used by the community to create all maps that exist xD. It's the successor of "Cube" and it's engine (Cube Engine -> Cube 2: Sauerbraten engine).
Cube 2: Tesseract (Freeware - Mac OS X, Linux, Windows), its a bit slower than sauer and based only on instagib, but still fun. Mapmaker Inside. It's the Successor of Sauerbraten and has an upgraded engine (Cube 2: Sauerbraten engine -> Cube 2: Tesseract engine).
Red Eclipse (Freeware - Mac OS X, Linux, Windows), It's a nice little arena shooter witch some gamemodes and many modifiers. Mapmaker inside. It's a fork of Cube 2: Sauerbraten.
And now some non-cube engine-game:
Blockstorm (Paid (1€ currently (on sale) - 5€ normally) - Steam only - Mac OS X, Linux, Windows), a very neat game. Mostly voxel based. It got player made skins for weapons and characters, ingame editor for skins and maps etc. Got a few nice weapons to play around with, and BADASS MECHAS! *EXPLOSIONS* with rockets and minigun.
For the Cube-games we can easily set up a server when if some1 got a root server. But in Blockstorm we could just open another room and a very high player restriction.