See also "Dumpster Dive".
Dumpster diving is also a very useful practice for much more than technology, as the "Dumpster Dive" node points out. A key point to success at dumpster diving is to dumpster places with high inventory turnover -- distributors, especially. To them, a box of something is bad because one item in it is bad, or because returning the whole box to the source is cost inefficient, etc. The risk is that if the distributor is large enough (perhaps they're a manufacturer too), they'll be guarded.
Although a distasteful idea to some, dumpstering for food is not such a bad idea when you dumpster distributors rather than restaurants or groceries. They will discard entire boxes of bananas when one bunch looks slightly spotty, while 50% of the box is still good. Also, distributors are likely to discard their foodstuffs in one dumpster, and their other trash in a separate one.
Bagel shops and bakeries are another good place, since their stock goes bad daily.
And just so you know you are in the presence of dumpster diving royalty: I have dumpstered beer.