Since people started using virtual hosts by name with Apache HTTPD and other web servers, it has become very difficult to figure out which virtual hosts live on a single IP, if all you have is the IP address.

Have a look at the Robtex Internet Swiss Army Knife. It solves this problem, and far more, including AS# lookups, BGP dereferencing, and DNS checks. There’s a firefox search toolbar available for the site (very useful!) and RBL (blacklist) check tools right on the main page of the site.