Contrary to popular perception, most dumpsters aren't gross and filled with rabies-laden rodents and rotting food. Some are, for sure, but those aren't the dumpsters that people dumpster dive in.
My college housemates are hardcore dumpster divers. They'd go every week, and bring their haul back to the house. They even teach a workshop on how to dumpster dive. They don't do it out of tightwaddery, but rather as a way to reduce waste and show the cool stuff that people throw out.
Also, at the end of the school year, the dumpsters become FILLED with useful stuff that students just throw out, probably because it's too heavy to bring on their flight home. During that time, they dumpster dive every day. I would have gone as well, if I had a car to lug stuff home in. I did nab a nice set of dishes, however.
My housemates are actually filming a documentary on dumpster diving on college campuses. Pretty cool stuff.
~mimi