xISBN is another web service we provide. It takes an ISBN of an item as input and returns ISBNs for other items in the same work-set. So, it is a way of finding ISBNs for multiple editions, translations or other versions of the same work.
One of the parties to use it has been Talis in their Silkworm initiative. In particular, they have a greasemonkey application which takes an ISBN on a page, augments it with alternative ISBNs using xISBN, and then carries out other actions on the ISBNs. See the walkthrough demo [Silkworm Threadster] and a more general presentation [ppt] - from which the above image is taken - discussing it.