WordNet

WordNet is a large lexical database of English, developed under the direction of George A. Miller, professor of psychology at Princeton University.

Word Net is a useful tool because of many reasons:

• Nouns, verbs, adjectives and adverbs are grouped into sets of cognitive synonyms (synsets)
• Each synsets expressing a distinct concept.
• Synsets are interlinked by means of conceptual-semantic and lexical relations.

The whole network of meaningfully related vocabulary items and concepts can be navigated through a browser

You can also  download a free version of the tool.

Note that the database is still under construction. But it’s a project worth being encouraged because of its importance for computational linguistics and natural language processing.