Though I've never been a huge fan of thesauri (someone put me off them years ago, and left me with the idea that if I can't find a word in my own brain it's probably not a good idea to use it) I'm very fond of The Word Menu. For those of you who've never seen it, it's a massive collection of words arranged not alphabetically but by category. I find myself turning to it whenever I need a job for a character (see Occupations/Jobs) or an interesting psychological condition to discuss (see Phobias) but mostly it's just great fun to leaf through when I'm feeling stale. Seeing pages and pages of words describing architecture, or states of being, or sex, or colors, never fails to remind me how interesting the world is.
You can find it here.
When I was poking around to see if the book was still in print (apparently so) I found out that it has an alternate life as a piece of software. This seems like a pretty good idea, and I imagine it would make an excellent alternative to Microsoft Word's thesaurus. (I used to worry that Word's horrible thesaurus would wreak stealthy damage on American prose, but it turns out that there are much worse threats.)
The Word Menu was assembled by a man named Stephen Glazier, who died shortly after the book was published. I have to admit this little fact disturbs me just a bit -- I think about the poor guy every time I pick the book up and wonder if somehow the massive effort contributed to his untimely demise (he was in his forties; it was a brain tumor). Perhaps one day I'll post about cursed books.