AoL (I'm ready to start the 6th palace)
Rainbow Six (the original on the N64) .. I just started a game up last night and got through the first couple missions.
I was playing SMB3 the last week or so too .. I forgot how frustrating especially the Pipe world is, then at the same time how painfully easy Bowser's Castle is after all the torture they put you through to get there.
I'm playing Max Payne ... I got up to RagnaRock (which is probably my fave chapter in the game) over the weekend. As many times as I've played it, it's still fun and challenging.
If you're into the stealth/action genre you might want to check out Splinter Cell if you haven't already. The newest one, Conviction, is exclusive to the 360 (and PC), but the first 4 are/were on all main consoles.