Cheat: EXP Exploit
If you are just short of an upgrade (e.g. you have 90k EXP and you need 100k), you can earn the amount you need, purchase an upgrade with EXP, and kill yourself and return to a checkpoint and you'll have both the upgrade and the 90k you had at that checkpoint, great eh?
An easy way to gain EP is on mission 22/31. After you steal the Black Ops helicopter, do not destroy the Hive when prompted. You can use the helicopter for as long as you want with its unlimited ammo. Switch to Infected Vision and blow up infected water towers to easily gain EP.
Unlockable Body Surf
To unlock a new ability called "Body Surf," navigate to the Cheats menu (under "Extras" in the Main Menu) and enter this code: RIGHT, RIGHT, LEFT, DOWN, UP, UP, UP, DOWN
Earning a Gold in every event unlocks Platinum Events, which are more difficult.
Unlockable: Hard Mode
Beat the game on Normal to unlock Hard Mode.
Unlockable: New Game
Once you beat Story Mode, you'll be able to play through it again with all of your powers and upgrades.
Prototype Unlockable: Secret Story Event
Complete all Scientist Consume events to reveal Karen Parker's location after her disappearance.