I wrote a post about the BookChaser Editions service last year, but encountered a reference to another service they offer, and this led me to others:

• BookChaser Covers : compares cover images available from Amazon, Google Books, LibraryThing, and Open Library.
• BookChaser BookInfo : compares information about a book obtained from Amazon, Google Books, ISBNDB, Library of Congress, LibraryThing, Open Library, and WorldCat.
• ISBN Analysis Tool : compares x-ISBN-like service availability for a given ISBN obtained from Amazon, Google Books, LibraryThing and WorldCat.

All lookup services are by ISBN.

The Internet Search Environment Number (ISEN) is a fantastic idea.  On-line databases would be assigned their own, unique number, similar to an ISBN or ISSN, which would identify them as a resource.  This would aid tremendously in organizing and ultimately accessing these troves of information that, for the most part, belong to the deep web.

found via Catalogablog