This should come as no surprise to you all...

[fireball3k]

I might be in the minority on this, but I thought Geist was a really good game. Same for 1080 Avalanche, if you like snowboarding games. Most of the other games I could recommend aren't exclusives, but I'll do it anyway: TimeSplitters 2 and Future Perfect are amazing if you like either GoldenEye 007 or Perfect Dark.

I don't know if you like Mortal Kombat, but the GameCube version of Deception is notable because it includes a few extra characters, like Shao Kahn. He's really underpowered in this game, but even so. Deception has a really cool Chess Kombat mode, where you play a VS. match if someone tries to capture a piece. If you like EA Sports games, SSX On Tour and NBA Street vol.3 both feature a few playable Mario characters. Also, Fight Night Round 2 on GameCube includes Super Punch-Out as a bonus, which is a SNES game, but it's still a cool bonus.

edit: completely forgot to say *which* Mortal Kombat it is that has extra content. Also added something about Fight Night Round 2. GameCube's got plenty of dudebro games in addition to classic Nintendo muscle from first-party games.