I seem to collect a few things. My most massive collection is books. I have shelves full of books.. mostly SF and some science/technology-related documentary or non-fiction (Stine's "Halfway to Anywhere", O'Neill's "2081", Mahaffey's "Atomic Accidents"). Some fantasy as well though. All of The Lord of the Rings, including Hobbit and Silmarillion. All of McCaffrey's "Dragonrider" Books. All the Dune books. All of Zelazny's Amber series. A whole bunch of Niven, Clarke, Heinlein, and Azimov. Terry Pratchett, Douglas Adams, and Piers Anthony are well represented. I've got the last few years of "Analog" magazine on the shelf as well (and all the other issues back to 1972 packed away in boxes in the garage). I've got 3 floor-to-ceiling bookshelves full of old-fashioned, dead-tree books in my bedroom and several more shelves-worth scattered around the house. I wish I had more space for a real book library.
I also have a large number of items related to deep-space science and exploration missions like Voyager, Viking, Galileo, MRO, MSL, etc. Mainly stuff like lapel pins and "baseball" caps, but even some things like event access badges for those that I was involved with. Also copies of press kits, a Mars Exploration Rover lunchbox, Lego sets, stickers, coffee mugs, a bunch of buttons.. Mattel did a "matchbox car" version of one of the Mars rovers and I snagged one. All sorts of space-related knick-knacks. Most of it is just in a jumble in boxes, but some of it - like the lapel pins - I have on display.
I've also got a small coin collection. Noting formal or valuable I'm afraid however. My grandmother was a traveller and I expressed an interest in some of the coins she had after one of her trips so thereafter every time she went someplace she brought me her pocket change. Probably well under $100 worth of well-circulated small-denomination coins from all over the world.