I collect plastic IC farecards. I originally only collected ones I bought abroad that I used when riding, but recently recanted this policy
PS: I need a Pasmo card.
I've got three Passnet cards hanging in my office from my trip to Tokyo, along with some JR-related keychains and cell-phone charms. And a map of central Tokyo with all of the rail lines plotted out that I stare at when I get bored. Otherwise, I limit my collecting to the models…