The Visual Thesaurus is worth a glance for amateur etymologists as well as writers. Linked node data visualization transforms the elderly thesaurus into a much more useful reference tool. Usage is fee-based but nominal.
Thesauruses (thesaurii?) identify context-appropriate and idiomatically correct synonyms. Apparently this isn’t so easy to for a search engine, Google included. A recent article from Wired.com went into a lot of detail about Google’s word search algorithms, and made the amusing disclosure that distinguishing between a “hot dog” and a “boiled puppy” is still an issue.