Quarters. I used to collect quarters starting the year before I went to college until just about a year ago, when it finally sunk in that I didn't need them for laundry since we have a washer & dryer in our apartment. Devin now collects state quarters, and so far has all of them that have been made so far.
This was seriously aided by a raid of Mike's piggy bank, Pistachio Pig.
Check out Ali's post on Q... I think it's my favorite Q so far.