Pagine Bianche (White Pages). This telephone directory also links to the Pagine Gialle or the Yellow Pages so you can locate businesses.
There are a few ways to use the site. The first is to click on ricerca, then plug in the surname and or address in chi/cosa and the comune(town, city or province) name in dove if known. Another way to use the site is to click on ricercha speciale and use it to do a reverse look up if you know a phone number but don't know who it's registered too.
No matter how you choose to use the site it's definitely worth checking out.