It's a semantic dictionary for English, drawing together words into lexical groups. These groups are supposed to represent mental groups that we all - in general - possess, as well as reflect the etymology of the word. The two aspects go hand in hand. There's also a Perl module to query WordNet.
Sadly, in its move from academia to real life the rhyming dictionary has lost the feature to find the path from one word to another. From Dirk I know that that kind of feature is very memory and CPU intensive, but it's still a shame. Me-mo: download source materials and implement myself. Yeah, one day.