Games with interesting co-op mechanics?

[StaceyPowers]

I was reading @Shagger talking about a game called Knack which was designed for kids, with a co-op role built in for parents or older siblings to help them out with tough sections, and @kingpotato and I both liked the concept.

I was wondering what other games you all are familiar with that feature interesting co-op mechanics?

[skyfire]

Looks cool to me. I think in near future we would find similar stuff on AR. 

[kingpotato]

Gyromite and Stack-Up from the NES