ISBN Links Tool
Here is a nifty little mashup (well, sort of a mashup I mostly wanted to include a link to wikipedia) I read about on Catablog. The ISBN Links Generator takes an ISBN and sends it as a search string to services such as Amazon, Google "Look Inside," Open WorldCat, etc. You get an error if the destination can't satisfy the query but otherwise it's a nice example of the advantages of a company making their APIs available. Plus, you get the cover art on the search results page to help you decide if you want tor read the book.