Co-op is something of a dying trend, unfortunately, and usually they're found in FPSs. Not only that, but those are also usually the funnest. I'd recommend Splinter Cell: Conviction, but I actually preferred the slower, more methodical nature of Splinter Cell Chaos Theory's co-op alot more. It's an original xbox title, but if you find a copy, you might find some awesome times come with it. Also, Earth Defense Force 2017 is a simple, fun, and insanely deluded little title that more or less serves as the gaming equivalent of B-grade cinema. It's you and a buddy guuning down giant ants, spiders, and robots with big guns. There's also Resident Evil 5, which is actually much better when played with a buddy. Co-op on Mercenaries mode is quite fun, as well.
Anyway, those are the ones I can think of off the top of my head. Anyone else got anymore?