This plugin allows you to add a thesaurus lookup to your sidebar, or anywhere else. Thesaurus service provided by words.bighugelabs.com.
I was shocked that in a world full of writers, there wasn't a simple thesaurus lookup tool. With the help of an API written by words.bighugelabs.com, this is that tool.
Very simple AJAX based interface, which allows for thesaurus lookups without leaving the page, so you can lookup a word before you lose the thought.
Designed for use in the sidebar, but could probably work anywhere.
Basic result limiting functionality is included, so you can return a maximum of N results.