Do you know any games that would be good choices for anyone who doesn't have a lot of time to spend gaming?

I find that the busier I get (especially during the school year), I don't really have time to spend even an hour just playing a game, so that makes it really hard to get into long and complex games. So I'm wondering what games there are that I could pick up for like half an hour between when I get home from school and have to run off again for some rehearsal or whatever. Maybe something with short levels, or no levels but not so complex that if you don't play it for a long time, you're not completely lost when you pick it up again.

I think FPS's would be good for that, especially if you play online. You can just hop on for however long and kill stuff with no worry of stopping in the middle of something. Anything else?