Probably the best freeware quake-like game. It's got it all;
Awesome graphics, tons of blood and some gore, 3 different player models, 7 weapons, about 10 gamemodes, about 50 different maps, and it even has a alien-mob-survival gamemode.
But best of all, it has a coop edit gamemode. You can literally make quality maps with the built in map editor.
Give it a try!
http://cubeengine.comAlso you could try out Cube 1, or Assault Cube.
Cube 1 is the original game that started it all, Assault Cube is a 'realism' mod to Cube 1 that was made into a standalone game. It includes realistic weapons along with gory deaths via the knife.