"Index of" "Name Last modified Size Description" "xxxxxxxxx"
Type that into Google, replacing the x's with something interesting, and you'll be able to browse index's of sites where the webmaster was too dumb to lock them down. People have found entire books (Harry Potter, Michael Crichton, etc.) and other interesting things. Myself, I accidentally stubmled across hundreds of SNES ROMs. I didn't take them, but if I wanted to, it was all open.
There are other Google "hacks" out there that are really cool. Some people have found Excel spreadsheets with bank account information! Webmasters could learn a thing or two about how to implement robots.txt properly!
Post some others if you find them
EDIT: The Register has an interesting article about "googledorking":
http://www.theregister.co.uk/2004/03/10/the_perils_of_googling/