From the creators at Princeton University at http://www.cogsci.princeton.edu/~wn/
 

"WordNet® is an on-line 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. "

I worked a lot with WordNet during my studies and used it as part of the research for my thesis.  I use this gigantic database of English words and relationships everyday with as much pleasure as the first time.  You can download it for free at the aforementioned site or look up words online.

Now also from your mobile phone!  I coded a WAP interface that allows you to get the WordNet information you want while on the road.  You need a WAP enabled phone (eg. Siemens S35 or Nokia 7110).  
One word lookup happens in three network round trips:

• WAP to http://www.esus.com/wapword.wml and type in an english word  WapWord retrieves the possible word classes and selection choices.
• select a word class (noun, verb, adjective, adverb) and a selection choice (synonyms, holonyms, meronyms, ...)
• WapWord retrieves the requested information.