I've been playing Killzone for the PS2 since I got it at Christmas. Love the game, especially the online version. And I recently found a couple of cheat codes for the single player version, Over Here at the KillzoneGame fansite.
One-hit kills
At any menu screen, hold L1 and press Circle, Triangle, Circle, Triangle, X.
All FMV sequences
At any menu screen, hold L1 and press Circle, Square, Triangle, Circle, Square.
Big head enemies
At any menu screen, hold L1 and press Circle, Square, X, Circle(2).
Every other website out there has the same stupid “cheats” ... i mean, tips on how to play, and what you can unlock when.