Now I have never actually gone into a dumpster, but I have found a few really cool things in stuff that people were going to just throw out. I have a few Pentium III computers that my father's company was going to throw out that he claimed. (We wiped the hard drives and reloaded Windows 2000 onto them.)
My favorite find so was in a pile of stuff in a garage of a co-worker of my father that was going to be thrown out. Deep inside the pile I found a green box full of rare, antique pre-war Lionel trains still in working order and really good condition. They had "no use for those old toys" so they let me have them.
