World At War. Downloaded it as a game on demand, since I'm waiting for New Vegas and Black Ops, like everyone is.
I didn't think I was going to like it, but I did. The respawning bads were annoying, and there were moments where you'd swear the AI is cheating just to frustrate you, but, as a challenge, I enjoyed it. It's not on the level of CoD4 for me, but, for someone who doesn't rate FPSs as their favourite genre, I still had a great time with it, and I couldn't put it down.