I'm a relative newcomer to the 360, having been a lifelong Nintendo devotee. But I love the 360 so far and I have to say, it's been getting a lot more play than my Wii of late (though Brawl has been keeping me honest).
Anyway, I guess with all of my experience with Nintendo, I've just learned to take local co-op for granted. So when I got my 360, I got a second controller and expected that there would be a lot of games my wife and I could play together, either against one-another or on Live. But it's been harder than I thought to figure out which games we can play together, especially online.
I've got LEGO Star Wars: Complete Saga (though we haven't started it yet) and Earth Defense Force, and I know those both have local co-op.
So what are your favorite local co-op games on the system? Also, when playing co-op locally, if both players sign in using their gamertags, do both gamertags get achievements unlocked while playing together?