A long time ago I was working on an outliner, and happened to come across the source text for a thesaurus and made a wonderful combination of the two, useful when trying to name products, for example. Or just when you want to play around with words.
Type a word into a headline. Say for example "friend." Double-click on the headline to expand it. The software would automatically fill in the synonyms for the word. Double-click on any of these words to get its synonyms. Pretty simple, and very useful.
I call your web service with one parameter, a word.
You return a list of synonyms.