[karolist]

Bing/MSN Search had this for ages in the form of query "ip:IP_OF_THE_SERVER" which would return a list of indexed websites having domains with this IP address.

I suspect this site uses Bing under the hood - finds the IP of entered website domain, queries bing and returns the results.

This is widely used by security folks when doing pen testing - even if you don't find any flaws on the targeted website, there's a great chance other websites won't be as secure if it's on shared hosting and such tools help to find those other websites.