I am addicted to Dictionary.com. I used to carry a Thesaurus around in my purse (well, really it is more like a suitcase) but now I have both at my fingertips at all times. I love that.
Refdesk, source for facts
Yahoo directory of reference sites.
Urban Dictionary is the slang dictionary you wrote. Define your world.
I discovered the photographic dictionary awhile ago, even sent them a peony photo for "p" submission but never heard back from them.
What are your favorite reference sites?