I've been playing Borderlands a lot with my boyfriend. Very humourous first person shooter. And I gotta say, I love shooting things in the face. XD
World of Goo was another fun one I played a while back. Also requires a bit of brain power. Some of the levels are a really good challenge.
If you like playing Worms, you'll love Gunbound (and it's freeeee!)
Champions Online recently went free to play, if you're into superheroes.
And Grand Chase is also fun and free. I need to get back into that one. lol