Right now I can only think of Splinter Cell: Double Agent, and arcade style Metal Slug Anthology have co-op. Both are quite fun in my opinion, should check em out.
the first thing that pops up in my mind when you ask that question is:
Resident Evil: Umbrella Chronicles x)
unfortunately its not gonnna be out until fall...
Given the design philosophy of the Wii is geared towards an "inclusive" or group oriented experience, I am very surprised that there aren't more co-op games available.
Maybe as new producers get on board they will make more.