All single player levels:
Select the "Cheats" option at the main menu, then enter happycamper as a code. 

All cooperative levels:
Select the "Cheats" option at the main menu, then enter wanderlust as a code.

All FMV sequences:
Select the "Cheats" option at the main menu, then enter filmcritic as a code. 

Infinite Ammo:
Enter the code PACKMULE in the cheat menu to unlock infinite ammo.