Oh man, I've found some insanely cool stuff in dumpsters.

Hubcaps to cars, all the time. That's fun.
Working Dishwashers.
Working push mowers.
Brand New Size 13 Timberlands (Wearing them now)
Coach and Prada Purses
DVD players (All the time. Usually just need cleaned)
13-19 inch tube TV's
Sleep Number Bed
Antique Silverware (1847 Rogers Bros Silver)
Surround Sound Systems (5 So Far)
Computers (Mostly intact, one I'm still using to watch Hulu)
Allen Bradley Automation Equipment (I made 600 off that)
Turkish Imported Rug (Sold for 170 at auction)
Hundreds of working Vacuums
Cell phones Galore

I don't really have an all time favorite. I've made bank off what I've found though. If I had to pick it would probably be the couch in my shop, it's one of those overstuffed black leather gigolo couches. I take naps on it all the time LOL.