Biblio.com

It was difficult to decide what to look at from the Web 2.0 awards list. First I went to Mango languages, but the free lesson in Spanish I tried seemed rather laboured even for a complete beginner. So then I plumped for Biblio.com and found there were 16 copies listed of a 1977 book called Where Beards Wag All. The Relevance of the Oral Tradition by Evans, George Ewart so it isn't as rare a book as I thought, obviously!
The search facility seems a bit clunky - if you type in the full author's name you get everyone with that surname even though you've specified one person. However, it does deliver the information you want and if the item you're looking for isn't there you can fill in a form and they'll look out for it for you. You can narrow your search in all sorts of ways to get just the right copy in the right condition at the right price.
This would be a useful site to recommend to a library patron who really wants a book that is out of print and not available in the library or through interlibrary loan.