Visual Thesaurus: A great way to find that perfect word
It's called the Thinkmap Visual Thesaurus. This program has an ingenious way of mapping the relationships between words. A regular thesaurus is fine, but seeing the words and their connections helps me far more than looking at a list.
You can head over to the Visual Thesaurus site and do a few searches for free to check out how it works. If you only need it a few times a week, then don't bother with a membership.
But if you get addicted, the cost is $2.95 per month, or $19.95 per year, after the 14-day free trial. And if you get a few friends together for a group membership, the cost goes down to $10 for the year.
If the price turns you off, you have an alternative in the Visuwords-online graphical dictionary. Using Princeton University's opensource database, it gives you a similar visual map with definitions and synonyms.
If you just want a "normal" thesaurus (and this one has a link to the Visual Thesauraus), try this site. And happy word hunting!