Musical instruments and Skye Terriers- real plus figureines and ephemera if it’s unique and good work.
I plan to soon begin a collection of Scottish Highland Cattle(live). That should be intresting…
As with many of you, my wife and I have books. Thousands, literally. We hired movers for our last move, and when they saw all the boxes full of books, they got surly: “They’re so damned heavy!” But since we don’t specialize in a particular kind of literature or multiple editions of works by a single author, I guess that’s not really a collection.
When on vacation, we keep lists of wildlife seen, especially birds.
But for real collections: I have lots of aerophones. Didges, whistles, flutes, eastern European and Asian wind instruments, Native American flutes, on and on. I used to play orchestral brass professionally (horn, trombone, tuba), and seem to take to wind instruments in general fairly easily.
I also have a lot – my wife would say Way Too Much – of juggling equipment. Dozens of juggling balls, many clubs, spinning plates, diabolos, devil sticks, shaker cups, yo-yos, balls for spinning on your finger, cigar boxes . . . just about anything that can be manipulated to neat effect. I have perfectly round, colorful, almost perfectly bouncy custom silicon juggling balls that cost over $60 each. We now have several boxes, a closet, and an entire wardrobe-thingy full of juggling toys, plus dozens of juggling-related videos and DVDs. I have a couple dozen fine kites, too, and used to be a sponsored flier (a maker or company would give me free products and support to compete or perform in various events at kite festivals), but I rarely fly anymore.
Gardening: I have a modest bonsai collection on which I spend a couple hours a week, and we have about twenty orchid plants of various types.
I am otherwise a fine, upstanding member of society. I think.
My wife: lanterns and candles, belly-dancing materials, and middle-eastern paraphernalia (she grew up in Saudi Arabia, so loves that music and decor).
I once tried to collect my thoughts - but failed 
Richard.
I have a small instrument collection - largely aerophones, but in particular, whistles, flutes and harmonicas. I also collect art and drafting supplies, pens (fountain pens, especially), and slide rules. Except for the instruments, most of my collections are family heirlooms. I hang on to them because they remind me of someone.
Most of my collections are small, though… I’m not allowed to collect big things 
Erik
Accidents…
you didn’t happen to live in atlanta at the time, did you? in the first apt my husband and i ever lived in, we had a brief, inexplicable, disgusting infestation of blue flies (those are the ones that mean something somewhere in the vicinity is dead, right?). i thought maybe between the rank smells from the kitchen upstairs and some old underwear, that might have explained it. LOL 