Sports!
Live, Madden, NFL 2KX, NHL 2KX, NBA 2KX, NHL Hitz, Tiger Woods, Links, and Top Spin. More casual gamers will prefer Top Spin over simulations like NHL 2KX and Madden.
Games like NBA Street vol 2 are also considered fun...heheh I hate the game.
Project Gotham Racing 2 is also very good, as well as Rallisport Challange 2 (from what I've heard). Moto GP1 and 2 are really good.
Of course there are a good number of FPS games.
Brute Force's 4 player is considered average by most, but I've had a lot of fun with it. Timesplitters 2 and Halo are the ones to get. Oh, and don't forget Deathrow!
DoA3 is great with 4 players. Kung Fu Chaos is alright, the minigames are the ones you'll spend the most time on.
Dare I say that the Xbox has dethroned the Dreamcast to become the #1 multiplayer console? It has all the sports games the DC has, all the fighters, all the racers...but adds one some awesome FPS games.