Exclusives:
-Super Smash Bros. and Super Monkey Ball have been mentioned already several times, but I'll mention them again. Both are must-owns.
-Pac-Man vs. One person plays Pac-Man on a GBA with a link cable while the other players play as ghosts. Best played with four players.
-Mario Golf: Toadstool Tour. You can taunt other players, or cheer them on!
-1080 Avalanche is worth a mention as well.
-The multiplayer in Mario Kart DD is great, but I'd suggest waiting for a price drop.
-Beach Spikers is a pretty good arcade style beach volleyball game from Sega, with digitalized babes.
Non-exclusive games with exclusive features:
Soul Calibur II: Thanks to the kind people at Namco, SSBM isn't the only fighting game where you can kick someone's ass with Link.
Non-exclusives:
-Burnout 2: Point Of Impact is the best racing game on the GC, and has some pretty good multiplayer options as well. Unfortunately, it's limited to 2 players.
-If anybody's looking for a puzzle game that supports up to four players, it's your lucky day - Zoocube does just that.
-007 Agent Under Fire supports up to four players. It's no Goldeneye, but my friends and I had fun with it. Unfortunately, Everything or Nothing's multiplayer options only support 2 players. I don't own Nightfire so I don't know how that is.
-I'm also going to throw in Sonic Mega Collection. Multiplayer Sonic 2 was awesome on the Genesis, and it's just as good here.