Coming from an older age, I was always used to using reverse DNS lookups and DNS transfers to figure out what sites are hosted on an IP. Transfers don't work on most server setups anymore and a reverse lookup is only going to give you one domain. I just found out that bing.com has a handy feature where you can search on ip:212.117.172.45 (replace 212.117.172.45 with whatever you want to search for) and it will show you all of the sites hosted on that IP address (or at least the ones that it knows about).
Here is an example link:
http://www.bing.com/search?q=ip%3A212.117.172.45
This is probably common knowledge for some and might even be on other search engines. I just noticed it today and figured I would share it since it seems useful for enumeration prior to attacking