Topic: 20 bucks

This is from corbie's review.
"If you have more than one player you can all join in and play the game together in a cooperative fashion. Each player gets their own colour of paddle, but since all of the paddles are on the same side of the screen it can be quite difficult to keep track of your paddle with all of the other players constantly moving over your paddle. You'll quickly find out that playing this co-op game can be more trouble than it's worth in the long run."

I think co-op works pretty well. Bit Trip Core is also co-op.

World of Goo and NyxQuest: Kindred Spirits (two of the best games on WiiWare) both feature two player co-op. WoG even supports up to four players.