I just bought my lathe & mill, so i haven't had a chance to build anything with the scrap I've found, so I don't know how well some things will machine. So far I have a couple of old flywheels that I want to make a large steady rest from. I have a long shaft from a hydraulic engine hoist, about 1.5" diameter & 30" long, & looks to be hardened, smooth & straight. I'm going to try to use it for lathe headstock, tailstock alignment.
Old car engines & transmissions have lots of good nuts & bolts, shafts & gears. I have saved a couple of car struts & a pair of leaf springs. I salvaged an old rack mount computer console, it had some nice pieces of 1/4" Almuinum plate, lights, fans, switches, & wire.
Keep your eyes open, & sometimes you get lucky. I found a couple of huge bolts half buried on the side of the road when I stopped to fix a flat, the bolts where about 18 inches long & 2 inches in diameter, I have no idea what they would have went to, a bridge maybe. I've run across stuff put in the woods before, but never tried to salvage any of it. Old cars, ovens, Old farm equipment has literally tons of good material on it. I also found an old school wood stove that was a couple hundred pounds of cast iron.I've found some good hardware being thrown out during/after a building remodel near my house, light fixtures, a bunch of little bags of bolts & screws, a chunk of counter top, a couple of ceiling fans. I got them all for free buy talking to the owner when I was walking the dog. Put the ceiling fans in the garage with 3 100watt CFL bulbs in them, really added some light in there, plus the 3 of the 4 four light fixtures worked fine, put those in the attic so I can see what's stored up there better.
I've gone to auctions, a few times, & it's amazing what you'll find. The longer you stay the better deals you'll get. I've also seen people go nuts at auctions & pay more for used stuff than what the same item would cost new at the store. Same thing at flea markets, at the end of the day you get amazing deals, some people will give you stuff just so they don't have to haul it back home.
By far the best scrounger I've ever seen is my girlfriends retired father. He's gotten a 30' x 50' steel building for free. The tire place was going to put up a new building & were getting bids to tear down there old building, so they could put up a new larger one. He goes & tells them that he'll do it for free, but they need it down in a day, he tells them it will be down by Monday. Of course they said yes. So him, his brother & 3 sons go up there & unbolt this steel building & hauled it away in a weekend. They were done on Sunday. It took a couple of hundred dollars in gas, to haul it back to his place but he got the roof, frame, walls, electrical boxes & lights too. It's a nice shop now.