ISBN
The ISBN module is not (right now) a CCK module but a module that can reference a CCK text field that is being used for ISBN data. The ISBN module will determines if the value entered is a 10 or 13 digit ISBN, verifies the check digit, calculates the 10 or 13 digit counterpart and stores this information with the node id in the database.
By itself this module has limited usage but a number of web service APIs can make use of ISBNs, including Amazon and LibraryThing's ThingISBN as well as paid services such as xISBN and Syndetics Solutions.
This module is still under development and needs to undergo many usability improvements. Anyone interesting in helping me with the module or building functionality on to the module, please let me know.