Games that used to be Playstation-exclusives that are now on Xbox 360:(Note: may not be a complete list)
1. Final Fantasy XIII (RPG)
2. Tales of Vesperia (Tales series) (RPG)
3. Grand Theft Auto IV (Sandbox)
4. Virtua Fighter 5 (360 alone has online play) (Fighting)
5. Devil May Cry 4 (Action)
6. Ace Combat 6 (Flight/Action)
7. Ridge Racer 6 (Arcade Racing)
8. Street Fighter IV (Fighting)
9. Tekken 6 (Fighting)
10. Soul Calibur 4 (Fighting)
11. Star Ocean 4 (RPG)
12. Fallout 3 (RPG)
13. Bioshock (FPS/RPG, already out in the market, PS3 version is delayed)
14. Lost Planet (360 first, superior textures) (Action)
15. Dead Rising (also on Wii, but the 360 has the definitive version in terms of graphics and gameplay) (Action)
16. The Last Remnant (RPG)
17. Guitar Hero Series (GH2, GH3, and Guitar Hero World Tour)
18. Resident Evil 5 (Third Person Shooter/Survival Horror)
Some of the more noteworthy Xbox 360 Exclusive titles:
1. Alan Wake (Action)
2. Ace Combat 6: Fires of Liberation (Flight/Action)
3. Banjo-Kazooie: Nuts & Bolts (Platformer)
4. Beautiful Katamari (Puzzle)
5. Blue Dragon (RPG)
6. Blue Dragon 2 (RPG)
7. Castle Crashers (Action)
8. Chromehounds (Mech Action)
9. Crackdown (Sandbox)
10. Culdcept Saga (Strategy RPG)
11. Dance Dance Revolution Universe (Rhythm/Music)
12. Dance Dance Revolution Universe 2 (Rhythm/Music)
13. Dance Dance Revolution Universe 3 (Rhythm/Music)
14. Dead or Alive 4 (Fighting)
15. Dead or Alive Xtreme 2 (Party game)
16. The Dishwasher: Dead Samurai (XBL Arcade)
17. Earth Defense Force 2017 (Action)
18. Every Extend Extra Extreme (XBL Arcade)
19. Fable II (RPG)
20. Forza Motorsport 2 (Full-on racing simulator)
21. Fuzion Frenzy 2 (Party game)
22. Fallout 3 - Downloadable Content 360 Exclusive (RPG)
23. Grand Theft Auto - Downloadable Content 360 Exclusive (Sandbox)
24. Gears of War 2 (Third Person Shooter)
25. Geometry Wars: Retro Evolved 2 (XBL Arcade)
26. Guilty Gear 2: Overture (Fighting)
27. Halo 3 (FPS)
28. Halo Wars (RTS)
29. Halo: Chronicles (??? Probably FPS)
30. Infinite Undiscovery (RPG)
31. Kameo: Elements of Power (Action)
32. Kingdom Under Fire: Circle of Doom (Action RPG)
33. Lips (video game) (Music)
34. Lost Odyssey (RPG)
35. Marble Blast Ultra (XBL Arcade)
36. Mutant Storm Empire (XBL Arcade)
37. Mutant Storm Reloaded (XBL Arcade)
38. Naruto: Rise of a Ninja (Action RPG/Fighting)
39. Naruto: The Broken Bond (Action RPG/Fighting)
40. Ninety-Nine Nights (Action)
41. Ninja Gaiden II (2008 video game) (action)
42. Pac-Man Championship Edition (XBL Arcade)
43. Perfect Dark Zero (FPS)
44. Project Gotham Racing 3 (Arcade Racing)
45. Project Gotham Racing 4 (Arcade Racing)
46. Project Sylpheed (Flight/Space shooter)
47. Raiden IV (XBL Arcade)
48. Ridge Racer 6 (Arcade Racing)
49. Rumble Roses XX (Wrestling)
50. Saints Row (Sandbox)
51. Scene It? Box Office Smash (Party)
52. Scene It? Lights, Camera, Action (Party)
53. Spectral Force 3: Innocent Rage (RPG)
54. Star Ocean: The Last Hope (RPG)
55. Tales of Vesperia (RPG)
56. Tenchu Z (Action)
57. Tengai Makyou Ziria: Haruka naru Jipang (RPG)
58. Too Human (Action RPG)
59. Viva Piñata: Party Animals (Party)
60. Viva Piñata: Trouble in Paradise (Pet simulator)
61. WarTech: Senko No Ronde (Fighting)
62. Zegapain NOT (Action)
63. Zegapain XOR (Action)
64. Rock Band 2 (Timed Exclusive to Xbox 360) (Music)
65. Splinter Cell: Conviction (Stealth)
66. Cry On (RPG from Hironobu Sakaguchi, the father of the Final Fantasy series)
67. The upcoming unknown 'Halo' game that was supposed to be announced at E3 but whose announcement was cancelled, because the Xbox division wanted to highlight the acquisition of Final Fantasy XIII. The next Halo game will be revealed extensively at an event called 'HaloCon' as reported by the gaming press.
Practically Xbox 360 Exclusive Games (out on PC but not getting released on PS3):
1. Mass Effect (RPG)
2. Gears of War (Third Person Shooter)
3. Bioshock (FPS)
4. Unreal Tournament 3 (FPS, 1080p only on 360)
5. Command and Conquer: Red Alert 3 (RTS)
The above lists are probably not absolutely complete since it's pretty hard to keep track of all these games....