Archivio Degli Edifici Abbandonati - A nicely designed site featuring pictures, maps and articles about abandoned buildings in Italy (in Italian).
Desolation W. Venezia - Artistic photographs of abandoned sites, like empty factories and flooded churches, near Venice (in Italian or English).
Lost and Least - A style-heavy site featuring photos of abandoned buildings, railways and military installations (mostly in Italian).
Napoli Below - A look at the tunnels and caverns beneath Naples (in English).
Subterranean Rome - Though the English version could be better worded, this site contains nice pictures and useful maps of the Christian catacombs under Rome (in Italian or English).
We Violate - A well-presented look at various sites, largely abandoned, around Italy (in Italian or English).