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 hi-res, print-quality versions of in-game posters (which you can then, obviously, print out), 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, but it's still a neat piece of marketing.
Collect Intel in Killzone 2, Get Rewards on Killzone.com [PlayStation]