Oh, Katamari is one sweet game. Can't wait for the sequel.
Mr. Mosquito (Ka in Japan) - PS2 - Awesome, awesome game. You're a Mosquito who has to stock up on blood for the winter. You harass a family throughout the game and watch their relationship deteriorate... so great.
Cubivore: Survival of the Fittest - GameCube - You're an animal (a cubivore) that has to eat its way to becoming the King of All Cubivores. It's the definitive quarky game. You even get to mate.
Adventures of Cookie and Cream - PS2 - Madness. You NEED two players to play it (unless you're just very skilled with a controller). Both players can use the same controller as you only need half of it to do tasks in the game. Cookie and Cream are on seperate sides of the screen and they have to help each other get to the end of the level without dying. Very fun.
I want to get Ribbit King, which looks good, but I haven't gotten around to it yet.