My collections tend to wax and wane over the years, and on more than one occasion I had to sell my collections for whatever I could get for them.
Shot glasses - I used to have so many of these, but sold them a couple of decades ago. Just recently started that collection back up, so I only have a few, but every time I go somewhere with a gift shop I'll look for one.
CDs - I used to have close to 1000 CDs but down to about 200 or so, but still buy them. Just a bit more picky these days.
Paperback books - I love going to a used book store and picking up old paperbacks. I sometimes get several versions of the same book with different covers. Mostly Sci-Fi, spy thrillers, military fiction. I also have a collection of old non-fiction military books, mostly focused on WWII. My dad gave me his collection of Ballantine's Illustrated History of WWII books, which I think is around 3/4 complete.
Rock T-shirts - I recently started buying more rock t-shirts at concerts or in the stores, and I'm starting to get quite a collection, well over 30 shirts right now.