The strangest thing I collect is earache from my wife 
HAHAHAHAHA
First of all, let me say this is a very interesting topic. There is nothing more interesting than having the chance to learn about the private obsessions of others.
I started with coin and postcard collection as a kid. Everybody I knew who was travelling abroad I would ask to give me some coins and postcards, and I would also buy postcards and obsessively keep aside coins when I travelled abroad. When I had three shoeboxes full of postcards a couple years ago, I decided it was pointless to "store" in place of "collect", and that it was an endless collection, so I gave it to my wife, who is a teacher, to take it to the classroom and show the kids many parts of Spain and the World.
The coin collection I still keep it, but most than half of it is stored in a bag, waiting for its turn to make its way into the display album. It is not a "numismatic" collection, coins are not collected by its face or numismatic value but just because they are from many parts of the World.
When I was 15 I got very much into Iron Maiden, and I would buy every IM lapel pin. I got about 35 different ones. Couple of months ago I put them on eBay for sale, just because it was a pity to throw them away. They sold for 5 euro...
When I got my electric guitar 18 years ago I also would buy any electric guitar lapel pin. I got about 30 too. This time I made a display frame and hung it on the wall. When the kid was born the spare room was for him, so the frame is stored in a box....
Also, I used to collect guitar picks. I have about 500. I also got tired of it. It was difficult to classify. I looked like a weirdo with a list on the music shops, because I couldn't remember which oned I had. "Pick Boy medium", "pick boy hard", "Fender medium tortoise" "Fender medium black". I think it was Ibanez the one who forced me to stop, releasing picks in about 10 different models with three or four different thicknesses, at .50 euro cents each.... some day I plan to make a big frame with all the picks, or put them in a display album. Now they are stored in a bag.
When I got into motorcycles, I also started to buy bike models. This time is not a "collection", I just buy the ones I like in real life, like if as I can't have all of them, at least I have the models. One day, at a Walmart in Nashville, I was in the toys section browsing for car and bike models, and I was looking in the Hot Wheels section and a 30-some guy asked me whether I collected Hot Wheels. I said no, I was just buying whichever looked cool. He told me he had thousands -can't remember how many-, he told me he had them hanging in walls, the whole room "tiled" with Hot Wheels models, each one different, each one in its original package. He explained me the Mattel codes and how to get the ones he didn't have. He also showed me his car in the parking lot. Sorry I can't recall which model it was, but it was a 60's model, maybe an Impala? He told me he has a son and that "he was making the collection for his son". I've heard that before, it seems to me as a way to give a reasonable explanation to an obsession.
