Even though am listing only my favorite ones.
World of Padman
Guess what? This game was made using Quake 3 engine. Just Imagine Quake 3 without terrifying looking monsters and the game support bots too, BTW the weapon isn't so scary looking but consider a toy-ish version of Quake 3's.
So if you don't want your friends to laugh at you just because they fragged you the entire hour, so this is your escape and if you practice harder, might have a chance on a come back if you've been trough that. :) Now, don't waste your time, go here: http://www.worldofpadman.com/ and download it now!
If you want more, there's a list of free FPS at Wikipedia you might want to check out. Happy fragging!
