The first bike I ever pulled out of a dumpster was Frenchy, a bright-red Gitane with the flashy foil wraparound decals. It was in the dumpster of the apartment building that Val and I lived in when we first moved to Denver. I never did use the frame for anything (it was too big for me) and it was hanging in the rafters of the garage when Val and I split up. All of the parts, even the steel wheels, found a use, though.
Since then, I have pulled quite a few bikes, or partial bikes, out of dumpsters.
I saw a handlebar sticking up out of a construction dumpster as I walked into the grocery store, one day. When I looked inside, there was a 1992 MB-3 Bridgestone mountain bike in there, minus its front wheel. I took it home, installed a set of wheels I had left over from a fixed-gear conversion, then sold the bike.
Once, as I rode to the coffee shop, I cut down the alley between Warren and Evans, and saw a wheel in the dumpster. The wheel was attached to a 1980 Schwinn 5-speed cruiser, with a drum-brake rear wheel. I rescued it, built a front drum-brake wheel for it, and installed a girder fork. It belongs to one of the baristas at the coffee shop, now.
Another time, I was walking up the alley to look at a garage sale, and I saw a handlebar in the dumpster a couple of houses down. It was on a crappy old department store bike, but the bar and stem were cool, vintage, French Tourist models. As I wheeled it back to to my truck, the guy at the yard sale said, "I knew I should have pulled that out and sold it!"
"Yep," I replied, as I strolled by.
Why do people throw good things away? I don't know, but I'm not too proud to rescue them!