Ok just heard about this lately, its actually rather cool.
goto live.com , in the search field enter;
<IP-ADDRESS> would be the IP of a webserver, adsl line or whatever, the neat thing is that you will now get a listing of all the domains hosted on this IP.. Nice..
Usually I use google for searching, and they may have a similar feature but I have not had time to dig into that.
What can I use that for you say? Well, say you want to find more information about the owner of a web-site, well there are all the usual steps like whois etc. but by using the above you can detirmine what other websites are hosted on that IP, this can maybe give you further info on the owner or the hosting provider.