I don’t have one handy at work, so I use Dictionary.com.
At home, it’s a different matter. I have
American Heritage
A 1963 Funk and Wagnalls (to see how words were used back then)
An Etymological Dictionary (dated from the late 1800s – the author talks about that brand new ongoing project, the OED)
Partridge’s Dictionary of Slang
*Descriptionary[/a] (sort of a reverse dictionary – look look up the description and it provides the word),
What’s What – Similar to the previous one, with photos.
Spelling Dictionary – with spell check, I rarely use this any more.
i]Le Mot Juste* (foreign phrase dictionary)
Translation dictionaries: English-French, English-Russian, English-Latin, English-Spanish, English-German
Brewer’s Dictionary of Phrase and Fable
And upstairs there’s the OED and a couple of cheap paperbacks.
In addition, I have a bunch of style guides: (Chicago Manual of Style, Washington Post Manual of Style, GPO Style Manual, Fowler’s Modern English Usage (original and revised), Strunk and White’s The Elements of Style, Quirk and Greenbaum’s Concise Grammar of Contemporary English, Karen Elizabeth Gordon’s The Transitive Vampire: the ultimate handbook of grammar for the innocent, the eager, and the doomed and The Well-Tempered Sentence.
There may be more somewhere.