Okito Thesaurus 0.5 (Mac)
Source: Okito Software
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 Okito Thesaurus you get a fast electronic thesaurus that automatically integrates with any service aware application including Okito Composer, Mail, TextEdit, OmniWeb, and more.