If you're into FPS's, one of the best shooters that I've played is Timesplitters. I'm not a huge fan of th FPS genre, but Timesplitters is quite excellent.
I first ran into it on the PS2. When I first got the PS2, I picked up a game from each genre, and TS was the shooter. It is fast. I mean really fast. The game never slowed, even when we did four player deathmatches on the console (thank you, Mr Multitap).
Then TS2 came out for the Xbox, I was all over it. Way, way better. Improved graphics, that kickass framerate, and even more player models, weapons, and locations. The story line on these games is pretty lame, but who cares.
If you're looking for a great game for parties, then this is it. Fun environments, fun weapons, and the only game where you can play as a monkey carrying a rocket launcher. Fear the monkey.
(yah, yah... Halo is great for party deathmatches, too, but it doesn't have monkeys)