I know this thread has been around for a while and a lot of ideas are already out there but I figured another two cents couldn't hurt.

A few things that sprang to mind when I thought of currencies: Cogs, marshmallows (no idea why), clothes pegs, someone mentioned sheets of music and it made me think of having actual music notes as currency (maybe a bit too abstract), feathers from a particular bird, marbles, belt buckles.

Going with the technology theme, keyboard keys could literally be keys for unlocking doors. Megahertz could be mistaken for some kind of weapon. A mouse could be mistaken for an actual robot mouse, assuming mice exist in the Knytt universe (and it has wheels instead of feet. Get it, mouse-wheels? ... Maybe not).