http://www.firearms-source.com/
FA:S is a game developed on the Source engine. Shoot bad guys, don't get shot, and do whatever the objective is. There are a lot of different (and mostly unique) (I'm looking at you, M16) firearms to choose from.
Keep in mind that this is an alpha, and there are quite a few bugs in the current version. The download is 1.5 GB and the game requires Source SDK Base 2007.
edit: we seem to be having about 25-30 players most of the time


