For 360, Call of Duty: Modern Warfare is awesome up to 4 players. Halo 3 is really fun when you mess with the gravity/speed settings, makes for some very interesting gameplay.
Worms is also a must, though I'd have to check whether that's a 360 game or just original Xbox.
Rock Band above all others - though it's expensive to get all the instruments. Soooo much fun.
Good luck and I'd also appreciate some recommendations for 4 players+! Here at the house 4 out of 8 of us (including myself) have Xbox 360s and multiple controllers.
Hooray for living with geeks!