Practically anything big enough to cut up for later use. plastic, Cloth,
Wood, Metal, Electronics although I prefer to buy new it save a lot of issues.
Except Tvs and other montiors. If you don't know what you are doing,
you can get a nasty shock from those (however, there is an instructable
about how to take them apart safely).
You are really only limited by your imagination. I've seen projects on here that run the gamut of materials used. There are common elements in some, but there is no foul in showing the community something new!