Sauerbraten is a free FPS similar to (but tops in every way) quake. It's the sequel to an older game called Cube.
It's still in development, but the latest release CTF contains I guess CTF in addition to the older modes and stuff (Never played before this release). It has both single player and multiplayer. Single player modes are SP Invasion and SP missions (not official names AFAIK, but that's pretty much what they are), and multiplayer has instagib CTF, Capture (which is cool, kinda lke onslaught in UT2K4 but capturing points is the only way to get new weapons), Free For All and Coopedit, I don't kow what that is, though.
Anyway, the game is great, it plays smooth and looks awesome.
Spoiler for screenies big =o :
Playing on singleplayer invasion on "Floating Castle" or something like that.
Spectating a FFA match.
Playing single player invasion on Thor, getting shot at by an enemy with a shotgun, I'm using the crossbow.
Here are the creators own shots, but most of them are from older builds.
http://sauerbraten.sourceforge.net/screenshots4.html The game also has an RPG mode which is still in development, and the only difference is that the game is played in 3rd person atm. (It's all one release right now, RPG maps are in DMSP under RPG)
The game has seven different weapons each with different perks.
Hand blade: You always have it, don't know the damage but it's high.
Pistol: Starting weapon, max ammo (100), damage (consistent 25). Low recoil.
Assault Rifle: Max Ammo (60), damage (30 per shot, aprox. 120 per second). Medium recoil.
Shotgun: Max Ammo (30?), damage (200 at point blank, when hitting multiple targets, the damage is divided between them). High recoil.
Grenade Launcher: Max Ammo (want to say like 12-20?), damage (60-69 I think, splash). Med-High recoil, bouncy!
Crossbow: Max ammo (I think 10), damage (100 even). Med-High recoil, like a rail gun, good weapon for picking enemies away.
Rocket Launcher: Max Ammo (I think 20), damage (120 on contact with enemy + splash to enemies around). High recoil.
It doesn't have any bots right now, but apparently there is a mod for them, but I think it's for an older release.
Ayway, the game is awesome. It's supports Windows XP, OSX and Linux. (Link to the site is at the top
)