Like every other shooter on the market, Killzone 2 features things that are hidden. Where Killzone 2 differs from the rest, however, is that your rewards for findings these are tangible.
How's this work? When you find in-game Helghast intel files, you can then visit the game's website and redeem real-world bonus items. Some of these are print-quality versions of in-game posters, some are random crap like Helghast wildlife files, while others take the form of papercraft.
Yup, Killzone papercraft. Pictured above. Won't win any awards for cuteness, and looking at those handrails, won't win any awards for accessibility, either!
Collect Intel in Killzone 2, Get Rewards on Killzone.com [PlayStation]