For me, anything magnetic. I don't get alot of heavy duty metals like greytruck described. For me I fill up 5 gal pails with small stuff, and just dump them in the pile...nuts, bolts, etc. Guys have seen me and never said anything. Or to make it easier I find something to fill up....filing cabinet, microwave
casings, etc. Then no dumping. Whole pile gets thrown off.
Also, grills, bicycles (basing the frame isn't aluminum) computer cases, futon frames...
If it has a small piece of something non-metal stuck to it they don't care. I had a shelving unit that had wood braces and a chunk of wood paneling on the back. Right in the pile. But you can't just fill up a washing machine drum with wood or plastic and expect them to take it