Wordnet is a freely available reference system that you can use to determine relationships between words. You could use this information to construct a hierarchy of relationships in a classification scheme. There are all kinds of related projects using the WordNet database such as Lexical Freenet which can be used to explore related concepts. The database can be downloaded for local use and a number of tools exist to help with development including code for Perl and Python. There is also a COM object available that can interface with the data.