I have a pretty large collection of Star Wars collectibles (figures, dolls, replicas, etc.), Transformers collectibles (toys mostly, but several other items) and Japanese hero collectibles (Super Sentai, Kamen Rider, etc). The majority of these were gifts, so they weren't collections I had intended to gather.
I have smaller collections of Digimon (toys, games, etc.), Green Lantern (mostly the comic version including ring replicas, figures, a LOT of comics, etc) and Dragons (although I do have several dragon items that also fall in other categories).
While I don't actively collect anymore, I also have a very large collection of Magic: The Gathering cards (including several that are quite rare from the first few releases) and I used to have a large collection of Star Trek items (sold most of them to pay medical bills).
I could probably sell off a few of these and have a fair bit of money to play with, but most items were gifts and have more sentimental value to me than they do monetary value. Those I've been willing to part with have pretty much already been sold to pay my medical bills when I was without insurance (and some since to help with the high costs I have even with co-pay).