Wikipedia has a special page Book sources.
Enter an ISBN, or pass it an ISBN in the format
and then you get a long listing of pre-constructed query syntaxes.
e.g. "Find this book at Safari, Amazon, in WorldCat, in RedLightGreen, ..."
The page is constructed from the templates on the regular Wikipedia Book sources page.
A similar idea to the bookservices macro in SnipSnap, but with a nicer format and huge list of potential places to query.
UPDATE 2005-05-10: In case you're not familiar with it, Wikipedia includes a built-in ISBN syntax that will automatically construct Booksources links for you.