Geez, where to begin?
Wooden coffee stirring sticks, which make great planking material in various scales. You can buy them at the store, or, if you're like me, you work in a building with a coffee service, and the guy who runs it is a nice guy and generous
Clear plastic packaging from food packages and blister packs, yes, but also, I save the clear plastic used in packaging dress shirts (make the collar stand up in the package). Good for windows in various scales, and also, I've made templates for scratching other parts.
Wire from old electrical appliances, like the cords on old lamps or other appliances. I salvaged some old speakers and got some very nice wire, plus big magnets. For that matter, guts from old radios (for us old-timers) or computers. I saw a very interesting scratchbuild of a futuristic tank, using an old computer mouse.
I have a stash of foil from candy wrapping, like from those miniature Reese's peanut butter cups. Folded in quarters, and laminated with CA glue, it's almost the perfect size for Prussian infantry flags in 54mm. I also use that, and old kitchen foil, for strapping, seat belts, etc.
Lead foil from dental X-ray blanks. Ask your dentist, they throw the stuff out, and it's becoming more of a pain to dispose of it. Mine gave me a bucket full of the stuff. Ask your dentist, too, if he's getting rid of any old dental tools, he might be willing to give you some.
I've taken to cleaning my used-up paint bottles, eg, Model Master. Cleaned and dried, they're great for storing other paint mixes (duh!) but also other things (chalk mixes, little bits of plastic and resin, very small rocks, etc).
I get rubber bands from the occasional newspaper that I get, and really thick ones that are used to bundle fresh vegetables (eg, asparagus, broccoli).
I belong to a theater, and we build flats (the walls you see on a set) out of pine stock for the framing, skinned with luan plywood (very thin, around 3/16), and we also use luan all over the stage. I come away from set construction and tear-down with a pile of scraps, good for bases or construction in small scales (eg, the walls of a house in 54mm). I also salvage some thicker plywood there, too.
Also from the theater, I work with stage lighting. New lamps (light bulbs, to the uninitiated) are packed in foam, which I use to pack figures into boxes. And we dress cables using zip ties, cable ties. They make very convincing .50 cal ammo feeds in 1/48 and 1/32 scale.
Landscaping materials come from the backyard, like roots dug up when I moved some bushes around. Dirt from the backyard, sifted and sterilized. Sand from the Jersey shore.
I save old candy boxes, ones that are around 2"x3"x6" or thereabouts, for packing/storing my 54mm figures. I've been experimenting with re-covering the lids with foil wrapping paper, to dress them up.
The Chinese takeout places started using these excellent plastic containers a couple of years ago, for their entrees, consisting of an opaque bottom and a relatively clear top. They come in both oblong and round shapes, and make great sorting/storage containers on my shelves. I also salvage the wire handles from the rice boxes, good stock for pinning parts to figures, and other uses where you need relatively stiff wire. The soup containers, by the way, I use for storing soup
Old twine, cut into various lengths and unravelled, makes good grass in various scales.
Old rust makes good....rust. One of the guys in our club is a machinist, he cleaned some machine at work and came up with a large can (around a half gallon) full of fine rust powder. We all filled as many containers as we could (like the old black plastic photography film canisters).
That's as much as I can think of, at this point.