Matteo,

do you mean an ontology-based "modeling framework" for modeling and building websites based on an abstract model like OOHDM, etc. (Daniel's pointers) or do you mean a library+API like php's symphony or something like the Drupal core, which can also be extended with modules based on callback hooks. There is sth. going on in the Drupal community you should look at: http://groups.drupal.org/semantic-web

I think it's best to re-use existing vocabs like FOAF, SIOC, DOAP, ... and add your own vocabs when you need new concepts.