The XBox 360 does NOT automatically play every old XBox game. You have to have a hard drive, and download the emulator to let it play properly. Here's the list of games that you can play. This list is accurate as of December 2005.

AirForce Delta Storm
Alias™
Aliens versus 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
Barbarian
Barbie™ Horse Adventures™ Wild Horse Rescue™
Batman Begins™
Battle Engine Aquila
Battlestar Galactica™
BMX XXX™
Brute Force™
Buffy the Vampire Slayer™: Chaos Bleeds
Cabela’s® Dangerous Hunts™
Cabela’s® Outdoor Adventures™ 06
Cabela's® Deer Hunt™ 2005 Season
Cabela's® Deer Hunt™ 2004 Season
Call of Cthulhu®: Dark Corners of the Earth™
Call of Duty™: Finest Hour™
Casino
Chicago Enforcer
Circus Maximus
Close Combat: First to Fight
Colin McRae Rally 4
Combat Elite: WWII Paratroopers™
Commandos 2: Men of Courage™
Conflict: Desert Storm
Constantine™
Crash Twinsanity™
Crash Nitro Kart™2
Crimson Skies®: High Road to Revenge™
Crouching Tiger, Hidden Dragon™
Dark Angel
Darkwatch™
Dead or Alive® 3
Deathrow
Digimon® Rumble Arena®
Dinotopia2
Drake™
Egg Mania™: Eggstreme Madness
ESPN MLS ExtraTime 2002
Euro 2004
F1 2001
Fable®
Fable®: The Lost Chapters™
Fairly Odd Parents: Breakin' da Rules
FIFA Soccer 2003
FIFA Soccer 2004
FIFA STREET
Fight Night 2004
Ford Mustang
Ford vs. Chevy
Forza Motorsport™
Frogger Beyond™
Futurama™
Fuzion Frenzy®
Genma Onimusha
Goblin Commander™: Unleash the Horde
Grand Theft Auto: The Trilogy
Grand Theft Auto: Vice City
Grand Theft Auto: San Andreas™
Gravity Games Bike: Street. Vert. Dirt.
Grooverider: Slot Car Thunder
Half-Life® 2
Halo®
Halo® 2
Halo® 2 Multiplayer Map Pack
Harry Potter and the Goblet of Fire™
Harry Potter and the Sorcerer's Stone™
He-Man: Defender of Grayskull
Hitman: Contracts™
House of the Dead 3
IHRA Drag Racing Sportsman Edition
IHRA Professional Drag Racing 2005™
Jade Empire™3
James Bond 007™: NightFire™
Judge Dredd®: Dredd vs. Death™
Jurassic Park: Operation Genesis
Kabuki™ Warriors
Kelly Slater's Pro Surfer
kill.switch
Lemony Snicket's A Series of Unfortunate Events™
LOONS—The Fight for Fame
Manhunt
Mat Hoffman's Pro BMX™ 2
Max Payne
Max Payne® 2
Medal of Honor European Assault™
Medal of Honor™ Frontline
Medal of Honor™ Rising Sun
Mega Man® Anniversary Collection
Metal Arms™: Glitch in the System
MicroMachines
Mike Tyson Heavyweight Boxing
Monster Garage™
MTV Music Generator™ 3
Murakumo: Renegade Mech Pursuit
MX World Tour™: Featuring Jamie Little
Namco Museum™
NBA LIVE 2004
Need For Speed™ Underground 2
NFL Blitz 2002
NFL Blitz 2003
NFL Blitz 2004
NHL® 2004
NHL Hitz™ 2003
Ninja Gaiden®
Ninja Gaiden® Black
Outlaw Golf 2
Outlaw Volleyball
Pariah™
Phantom Crash™
Pinball Hall of Fame™
Pitfall®: The Lost Expedition™
Predator Concrete Jungle™
Prince of Persia: The Sands of Time™
Pro Evolution Soccer 5
Pro Race Driver™
Pump It Up™: Exceed
Pure Pinball
Puyo Pop Fever2
Quantum Redshift®
Rayman Arena
Raze's Hell™
Red Dead Revolver
Red Faction® II
RedCard™ 2003
Robotech: Battlecry
Rocky™ Legends
Rogue Ops
Samurai Jack
Samurai Warriors™
Scooby Doo!™ Night of 100 Frights
Scrapland
SEGA GT™ 2002
Shadow The Hedgehog™
Shamu's Deep Sea Adventures
ShellShock: Nam '67™
Sneakers™
Sniper Elite
Soccer Slam
Sonic Heroes™
Sonic Mega Collection™ Plus
Speed Kings™
Sphinx and the Cursed Mummy™
Splat Magazine Renegade Paintball
SpongeBob SquarePants™: Battle for Bikini Bottom
SpyHunter® 2
Spyro™ A Hero's Tail
SSX 3
Stake™
Star Trek: Shattered Universe
Star Wars®: Episode III Revenge of the Sith™
Star Wars® Jedi Knight®: Jedi Academy™
Star Wars: Starfighter™ Special Edition
Star Wars®: Knights of the Old Republic™
Star Wars® Knights of the Old Republic® II: The Sith Lords™
Street Racing Syndicate™
Stubbs the Zombie™ in Rebel without a Pulse™
Super Bubble Pop
Super Monkey Ball™ Deluxe
SX Superstar™
Tecmo Classic Arcade
Teenage Mutant Ninja Turtles™
Test Drive®: Eve of Destruction
Tetris Worlds™4
The Great Escape
The Hulk™
The Incredible Hulk™: Ultimate Destruction
The Lord of the Rings™: The Return of the King™
The Simpsons™ Hit and Run™
The Simpsons™ Road Rage
The Terminator™ Dawn of Fate
The Thing™
Thief: Deadly Shadows™
Tom Clancy's Ghost Recon
Tom Clancy's Ghost Recon™ 2
Tom Clancy's Ghost Recon® 2 Summit Strike
Tom Clancy's Rainbow Six® 3
Tom Clancy's Rainbow Six® 3 Black Arrow™
Tom Clancy's Splinter Cell®
Tom Clancy's Splinter Cell® Chaos Theory™
Tom Clancy's Splinter Cell® Pandora Tomorrow™
Tony Hawk's American Wasteland
Tony Hawk's Pro Skater™ 4
Tony Hawk's Underground 2
Tork™: Prehistoric Punk™
Toxic Grind
Ty The Tasmanian Tiger™
Ty the Tasmanian Tiger™ 2: Bush Rescue™
Ty the Tasmanian Tiger™ 3: Night of the Quinkan
Urban Freestyle Soccer
Vexx™
Volvo: Drive for Life
World Series® Baseball 2K3
Worms 4 Mayhem
Worms™ Forts: Under Siege
WWE™ Raw™ 2
XIII
Yourself!Fitness™

For an up-to-the-second list of the titles currently supported, visit the Microsoft XBox 360 Website