I am not sure if other people know this or even how long it has been around, but I just noticed it.
On Bing you can do an ip search
for example seeing the other websites on this forum's ip would be ip:184.108.40.206
Some disclaimers would be
this won't find every website on the ip.
this isn't a server based search just an ip based eg some of my servers have dozens of ip's and single site on 1 ip and then dozens of sites on another ip.