limited-edition t-shirts
  limited-edition action figures
  dinner with the devs
  cool gold-colored cd/dvd/usbkey/etc with the software
  your likeness as a minor character
  limited-edition plush toys

Basically any kind of promotional tchotchke you can think of. It would have little intrinsic worth but it would be RARE which means that if the product became well-loved it would be worth something down the line when you decided you no longer had room for it - and until then it's an item that tells people something about you. "Oh man is that the commemorative bong for Psychonauts 2? Holy cow I loved that game."