Yes! I'm a very big picker-upper, I'm afraid. I have a compartment in my purse that's usually full of little things...sequins and confetti, buttons, little bits of machine parts, ribbons and bits of jewelry. I stick a lot of it in my art journals.
The biggest thing I ever 'just found' was a guitar in a trashcan at a boot fair (US people, that's kind of like a swap meet, where people sell stuff out of the trunks of their cars). I took it home, but it had gotten warped with the damp and I couldn't play it, so I took the strings off and kept it to make an art project out of. Still have it somewhere. I've also found (and worn!) a hat, scarf, and a pair of Hello Kitty shades. And I have a journal I found - clean and mostly empty, apart from a scribble and a swear word written inside the front cover! - in the public restrooms in Volunteer Park in Seattle. (I filled it with my own stuff because I always journal when I'm traveling and as luck would have it, I'd just completed one!)
However, if we're talking about just general collecting...I've acquired a huge range of junk. Among it, the following:
Books, both the kind I buy to read, and old ones, like dictionaries, kids' primers and ABC books, 'housewife' type guides...I like to cut them up and use them for collage, but I also flick through and read some of them - can be quite amusing. And old medical books, which have gorgeous artwork.
Magazines, programs and pamphlets
Postcards, old envelopes, telegrams and similar ephemera
Catholic votive cards (I'm not a Catholic, I just like the artwork)
Beads, buttons and shiny things
Rocks, shells, fossils, crystals
Dolls and bits of dolls
Tarot cards
Magazine cuttings
Art and craft materials generally...rubber stamps and especially stick-on lettering
Stickers
CDs
Anything Seattle related
Buttons with amusing slogans (I have them all up at work)
Anything connected to Nirvana
TrueDave, I couldn't resist keeping the wisdom tooth I had out the other year, and the oldest thing I've kept (from my life) is probably my baby shoes, which I still have in a bag somewhere...
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"Grunge? Isn't that some gross shade of greenish orange?"