Microsoft have released a list of original XBOX titles that will be playable on the XBOX360. The full list can be found below.

Note: A software emulator is required for each original Xbox game you play on your Xbox 360™ console. Please check back for more details as we approach the launch date.

Airforce Delta Storm
Alias
Aliens vs. Predator: Extinction
All Star Baseball 2003
Amped: Freestyle Snowboarding
Army Men: Sarge's War
Atari Anthology
ATV Quad Power Racing 2
Baldur's Gate Dark Alliance II