Google Hacks Video Halla from Information Leak published three videos show people how to put some useful strings and find interesting pages.
Basically, it's using application specific + unique data embedded in the URL or HTML to find sites uses those applications. The search string find other people's security camera backend is creepy(and cool) :yes!:
(Hacking into someone's FTP is also easier with Google's help)

Check out these three

Google Hacks 1
Google Hacks 2
Google Hacks 3