What: wordnet

 Where: http://wordnet.princeton.edu/ 
 Description: Online lexical reference system.
        A lexical database for the English language
 Updated: 12/2006
 Contact: mailto:[email protected] 

That is, a dictionary.

There is an online version [L1 ] and an off-line version [L2 ]. The off-line version is avalable for Windows and Unix.

It is mentioned here because it is based on Tcl/Tk. Don't expect too much. It works, but is rather poorly designed.