A few recent FREE demos are available on Live for the 360. All are worth checking out:
The Outfit - hybrid 3rd person RTS, demo is live multiplayer only. Good fun.
Burnout Revenge - 'nuf said
Blazing Angels - This demo just went up on Wednesday. WWII flight game. Graphics are UNBELIEVABLE. Esp the third mission over London. It looks like a freak'n oil painting. AMAZING. Also, the german one liner's are great as you hit 'em. tip: use your squadron commands.