Get it here
Everyone needs to get on this one. It's a free online first person shooter, and just pure deathmatch. Up to 16 players can jump in, and you can have up to four teams.
It runs on my old crappy computer with no problem (on the lowest settings, obviously) and is real nice for just killing time.
Little note - if it takes a while to load up a server, don't get discouraged. It'll happen, it'll just take about two minutes to get in completely.