I collect unmatched socks! I usually add to my collection every laundry day. I used to throw them out, but I would always find the mate the next week. Now I save them, and never find the mates!
I've managed to fill every spare space in my house in the last 25 years, so my coffee mug collection, tin collection, and book collection have all been packed up and stored in my attic.
Someone has already said this, but now all I have room to collect is memories. They take up so little space.