www.xbox.com/en-US/Live/countdown2011/home
For the 2011 countdown, xbox has discounted all of the sonic arcade games that it has. Personally, I downloaded Sonic 1, Sonic and Knuckles, and Sonic Adventure all for ten dollars. Quite a good deal if you ask me.
I picked up Sonic Adventure since it was only $5. Didn't really bother with the other games since I already own them.