Graphwords (http://graphwords.com/) is another thesaurus visualisation tool that uses Wordnet (http://wordnet.princeton.edu/). Type in a word and it generates a map of associated nouns, adjectives, verbs and adverbs.

To view the meaning of a group of words move your cursor over the node and to explore a word and its related terms in more detail simply click on it.

Many thanks to Carol Bream for the alert.