I used to collect tshirts from places I'd been, but I gave a lot away recently.
Currently I collect:
-Tea/coffee mugs from places I've visited (as an avid tea/coffee drinker, you really can never have enough, especially since they break)
-Foreign money (again started when I began traveling, I figure I can always exchange it back if I need to)
-Tri Sigma badges (okay collecting is a bit of an overstatement as I have two, but this collection will hopefully grow soon)
-Mementos (This is my least useful but largest collection by far. Think every program/ticket stub from most every movie and play I've pretty much ever been to, old notes people have written me, special cards--I finally threw out every generic birthday card I've ever gotten-- little tokens from different places I've visited. It's seriously out of hand. Think a four foot by three foot by two foot drawer in my room-- that's 24 cubic feet of crap. I keep thinking that eventually I'll make a scrapbook out of it all, but it'd have to be several for all the stuff I have.)
-Boy's hearts-- I kid, I kid.