A first promising one:

http://www.sensocomune.org

I think that a good choice can be BabelNet based on Princeton WordNet and various Wikipedia. It includes italian but also English, French, Catalan, Spanish, German. There is an API, I you know Java it is easy to use and there is a forum where users can help you.

http://lcl.uniroma1.it/babelnet/

Take a look at Marco Baroni's list of tools: http://clic.cimec.unitn.it/marco/tools_and_resources.html

Also, if you're looking for corpus data, there's itWAC, also by Marco: http://wacky.sslmit.unibo.it/doku.php?id=corpora

Moprh-it , a great italan lemmatization tool

http://dev.sslmit.unibo.it/linguistics/morph-it.php

Today at University of L'Aquila, Dipartimento di Scienze Umane, we discussed the Master degree Thesis by Pamela Naddeo "I nomi collettivi derivati in Italiano: analisi diacronica" with reference to the recent MIDIA Italian corpus

http://www.corpusmidia.unito.it

Last week in Pisa I attended the First Italian Conference on Computational Linguistics, CLiC-it, were many Italian language corpora and tools have been presented.

Open access Proceedings are available at the event web site.

http://clic.humnet.unipi.it