On a cycling forum I'm on, I've seen a few posts where someone noticed a nice frame or bike by a dumpster or trashcan, and notified other members.
It's amazing how many people can't tell if a frameset with a bit of rust is a high end or low end frame.
Personally, I've only taken two things out of a dumpster, both of which I had immediate uses for. One item (a PC) probably didn't count, since it was at a non-profit institution where I was notified by an employee who, due to the policies in place, couldn't easily donate the older machines, but could throw them out. <_< The other was a light fixture I could use when I walked past a place that was being remodeled.
The problem I see with dumpster diving is it is easy to think that the items are "free", when everything you own requires you to maintain it and store it. Some people don't understand that, and they end up stockpiling items of dubious value that they might use one day, and sooner or later the cost of storing those items destroys their lives.