Bonus content codes - At the main menu, select the "Extras" option. Choose the "Bonus Content" option, then enter one of the following codes to activate the corresponding cheat function:

Urban Combat single player level - Enter "sp-street" as a code to unlock the Urban Combat single player level.

Village Assault single player level - Enter "sp-village" as a code to unlock the Village Assault single player level. -From: John Searles

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Darkness Falls (15 points): Complete the first mission on normal difficulty or higher.
Anvil (15 points): Complete the third mission on normal difficulty or higher.
Captains of Industry (15 points): Complete the second mission on normal difficulty or higher.
Graveyard (15 points): Complete the fourth mission on normal difficulty or higher.
The Mountain King (15 points): Complete the fifth mission on normal difficulty or higher.
Living Quarter (15 points): Complete the sixth mission on normal difficulty or higher.
History Repeats (15 points): Complete the final mission on normal difficulty or higher.
Bull's-eye (90 points): Complete the first or second half of a mission with an accuracy higher than 50%.
Top Scorer (50 points): Be the player with the highest score on the winning team of a ranked match.
Killing Spree (50 points): Score 5 kills in a row without dying in multiplayer.
Rampage (75 points): Score 10 kills in a row without dying in multiplayer.
Flawless (50 points): Win a ranked match without dying.
Marksman (75 points): Finish a ranked match with an accuracy level higher than 40%.
Darkness Falls - Stopwatch (25 points): Complete the first mission in under 15 minutes.
Anvil - Stopwatch (25 points): Complete the third mission in under 25 minutes.
Captains of Industry - Stopwatch (25 points): Complete the second mission in under 25 minutes.
Graveyard - Stopwatch (25 points): Complete the fourth mission in under 50 minutes.
The Mountain King - Stopwatch (25 points): Complete the fifth mission in under 30 minutes.
Living Quarter - Stopwatch (25 points): Complete the sixth mission in under 25 minutes.
History Repeats - Stopwatch (25 points): Complete the final mission in under 40 minutes.
Bulletproof (90 points): Kill 100 hostiles without dying in the first or second half of a single player mission.
Darkness Falls - Ironman (20 points): Complete the first mission in a single session without dying.
Anvil - Ironman (20 points): Complete the third mission in a single session without dying.
Captains of Industry - Ironman (20 points): Complete the second mission in a single session without dying.
Graveyard - Ironman (20 points): Complete the fourth mission in a single session without dying.
The Mountain King - Ironman (20 points): Complete the fifth mission in a single session without dying.
Living Quarter - Ironman (20 points): Complete the sixth mission in a single session without dying.
History Repeats - Ironman (20 points): Complete the final mission in a single session without dying.

Additionally, there are two secret achievements:

Noob (0 points): Suicide 10 times in a multiplayer match.
Fashion Police (100 points): Knock off 20 hats in a multiplayer match.