Google Hacks has an article based on a presentation at blackhat Vegas about Google hacking. Google can be used to identify vulnerable servers, find exposed password files and anything else you might be looking for. Thanks to the Google cache, you might have access to things that are no longer even on the site.
Security Focus ran an article on this back in April that is a bit more technical than the article.
GoogleDorks is my favorite site for this sort of thing. It a collection of searches that people more imaginative than me have used to find vulnerable servers.
Google is the hackers first step in network reconnaissance. Be aware of what you are making available.