WordNet(r) is an online lexical reference system whose design is inspired by current psycholinguistic theories of human lexical memory. English nouns, verbs, adjectives and adverbs are organized into synonym sets, each representing one underlying lexical concept. Different relations link the synonym sets. WordNet was developed by the Cognitive Science Laboratory at Princeton University under the direction of Professor George A. Miller.

This is a strikingly thorough way to look up and fully explore a word. It differs as much from your standard dictionary as a rhino differs from a telephone.

As download squad puts it:

"In other words, it might help your writing instead of hurting it (like a thesaurus tends to do)"

In addition to the online version, there are downloadable versions for Windows, *nix, and Mac.