In the main menu, select Options, then Extras, then select the "Enter Code" button. Enter a code from the two below here to unlock it's respective reward.
- MAPMAYHEM: Bonus Multiplayer Map Pack (4 new Wrecking Crew maps)
- HARDHITTER: Unlocks Gold Sledgehammer for use in singleplayer
You can also unlock cheats for singleplayer use. These are unlocked by collecting certain Achievements/Trophies in Red Faction Guerilla.