Amazon Image Services
Amazon serves lots of images of book covers. Nat Gertler decodes the URL scheme, which could be useful for people designing web services. He provides thoughtful guidelines for using this service considerately.
For example, the Tinderbox macro ^do(amazonCover,ISBN) is easy enough:
<img align="right" src="http://images.amazon.com/images/P/$1.01._PE00_TZZZZZZZ_.jpg">
The macro takes your ISBN number and plugs it into the URL, replacing the placeholder $1. Once you've typed the macro, you can forget about the mysteries of the URL scheme and retrieve a clean scan of almost book cover whenever you want.