I learn this unique feature at Microsoft latest search engine, Bing.
You may perform reverse lookup with an IP address for a main site and generate a list of sub-site with Bing search engine.
For example, you enter "ip:126.96.36.199" (without the quote). This is the IP address for SlashDot main. It will results in 310,000 answers with a list of sub-site that hosting at the same public IP address.
This is a great feature as it can allow a pentester to find out how many websites are hosted at one particular IP address during reconnaissance. Cool!