for Xbox 360:
4+ player games -
Bankshot Billiards (up to 3 players, or you could do teams I suppose)
Dead or Alive 4 (you can do 2 on 2 tag teams... it can be really fun)
Fuzion Frenzy 2 (up to 4 players, all simple party games)
Marvel: Ultimate Alliance (up to 4 players on the same screen kicking ass with super heroes)
Monster Madness: Battle for Suburbia (4 players on the same screen classic arcade beat'em up gameplay with modern graphics)
Rumble Roses XX (most people say this is a shitty game but it can be quite fun with a group of people)
Small Arms (4 player multi level brawler... think smash bros with long range weapons)
Teenage Mutant Ninja Turtles 1989 Classic Arcade (the classic 4 player arcade game)
Wik: Fable of Souls (it's kind of a weird game, you might want to download it first
Worms HD ('nuf said)
WWE SmackDown vs. RAW 2007 (4 player wrestling... what could be better)
2+ player games that are good for parties
Smash TV (classic arcade co-op gameplay)
Street Fighter II Hyper Fighting (classic arcade gameplay)
Rockstar Table Tennis (2 player but short games make it good for tournaments)
Ultimate Mortal Kombat 3 (classic arcade gameplay)
There are also a number of 4 player sports games and FPSs but those are only good at parties if the players like those kinds of games.