Did you think the search engine wars were over, and Google won? New kid on the block Blekko, currently in closed private beta, urges you to reconsider. Once out, it's bound to become the favorite of search refiners, with the results likely made available as an API.
The Big Huge Thesaurus API provides a convenient interface for initiating a search based on an English word and retrieving related words and phrases.
The Big Huge Thesaurus includes more than 145,000 english language words.