Xbox 360 finally got a backwards compatible update! The biggest game included IMO is Mortal Kombat Armageddon. Shenmue 2 also.
http://www.xbox.com/en-US/games/backwardcompatibilitygameslist.htm
New titles
All-Star Baseball™ 2005
Aquaman: Battle of Atlantis
Buffy the Vampire Slayer
Dead Or Alive Ultimate
Destroy All Humans!
Dynasty Warriors 4 / Shin Sangokumuso3
Evil Dead: A Fistful of Boomstick
Fahrenheit/Indigo Prophecy
Far Cry: Instincts
Incredibles/ Mr. Incredible (J)
I-Ninja
Leisure Suit Larry: Magna Cum Laude
Mortal Kombat: Armageddon
MotoGP
MX Unleashed
Open Season
Pariah
Psychonauts
Rainbow Six: Lockdown
Rapala Pro Fishing
Scarface
Shark Tale
Shenmue II
Splinter Cell: Double Agent
The SpongeBob SquarePants Movie
Ultimate Spider-Man
Wakeboarding Unleashed: Featuring Sean Murray
Whacked!
Winback 2: Project Poseidon
Xiaolin Showdown
Updated Games
Burnout® 3: Takedown™