Hmmm... I collect a few things, although I'm not sure if all of them would really qualify as collections, because they're kinda generic. But I'll list them anyway
Manga & Anime DVDs
Hello Kitty Merchandise
Gemstones (Precious & Semi-precious stones, but not jewellery)
Alice & Wonderlands (different copies/editions)
Anime Plushies/Figurines/Mugs
Retro Cartoon DVDs (He-Man, Thundercats, etc)
Pear Ornaments (NO idea how it started...)
Baseball Boots (aiming for every colour ever seen by man or beast)
Skull accessories/ornaments/merchandise/anything
Used to have a lot more collections that were ACTUAL collections, but when I got my house, about 95% of my belongings never made it from my parents house to mine, so everything kind of started from scratch. Nontheless, I've built up some pretty decent collections in the past few years. Even if I am having to re-find many things I used to have before.