Nisus Thesaurus (Free)

From the developers website:

Mac OS X comes with built-in spell checking for many applications, but why stop there? Now you can have an integrated thesaurus as well. With Nisus Thesaurus you get a fast electronic thesaurus that automatically integrates with any service aware application including Nisus Writer Express, Mail, TextEdit, Safari, and more.

Using Thesaurus is easy. Just select the word in your favorite application and choose "Nisus Thesaurus" from the "Services" sub-menu. When you select a word in Nisus Thesaurus, the synonyms for that word will be shown right next to it in the Word Browser. Go from "sky" to "cumulonimbus" in just a few clicks.

Get it here.