I'm currently about half-way through Earth Defense Force 2017, and I can't recommend it enough. It's an almost stupidly simplistic third-person shooter, but it's strangely addictive, and the enemies are larger than life. It's extremely fun in co-op, too.
Also: The Orange Box is a must-have, if you don't already have it for PC.
As for XBLA, my favorite download is Pac-Man Championship Edition, which is great to pick up and go a few rounds with for 10-15 minutes on a whim. Also remember to grab Aegis Wing, since it's free.
"'Casual games' are something the 'Game Industry' invented to explain away the Wii success instead of actually listening or looking at what Nintendo did. There is no 'casual strategy' from Nintendo. 'Accessible strategy', yes, but ‘casual gamers’ is just the 'Game Industry''s polite way of saying what they feel: 'retarded gamers'."
-Sean Malstrom
