Here are all the cheats
whysoserious- Unlocks Silly Mode. (Countdown voice changes and the "Reload" icon changes to "Pimpmobile")
chrisbingbong- Unlocks "Bing Mode" (Game title changes to Bing, Boom, Splat! and you play as the evil director from Spares)
telford- Unlocks unlimited ammo.
callumhowarth- Unlocks all of the levels.