Since yesterday, Xbox 360 owners can download the free-to-play version of Defiance, the same version by Trion Worlds that was available on PS3 and PC. All you need is an Xbox 360 Gold membership, and you're ready to go.
If you play Defiance now, as the game is based on the Syfy show of the same name, you'll be in time for the aftermath of Season 2's finale. You'll be able to experience the crossover storyline of the game with the upcoming Aftermath update that will arrive next week.
Defiance owners will have several advantages over their free-to-play counterparts including more character slots, inventory slots, and loadouts.