When I need the spelling or the definition of a word, I usually use my Merriam-Webster Dictionary Firefox addon. But when I saw Definr demo-ed at the "Have You Heard?" program at the WAPL conference I was wowed!
Definr is a single-page, super-fast dictionary lookup tool that suggests words as you type.
Start typing a word, and definr uses their word-completion algorithm to suggest possible word matches as you add letters.
To make it even more useful, you can add the Definr search extension to Firefox, and add a Definr link to your Firefox bookmark toolbar; go to the Definr Tools page for details.
Definr's free dictionary is based on Princeton's open WordNet 2.0.
I could have used this tool a few months back when I had trouble figuring out how to spell the word "apropos" (as in "apropos of nothing"), and when Firefox's built-in spell checker didn't have it, and after trying every spelling alternative I could think of, I IM-ed a colleague who knew how to spell it. (Thanks again, Karla!)