Evergreen, oISBN
Dan Scott and Mike Rylander, who work on Evergreen, the open source integrated library system, pointed me at some discussion about oISBN: “I thought I’d go ahead and plug our xISBN-like service, called oISBN, that exposes the metarecords that our MARC fingerprinting algorithm creates.”
The link in the old blog post doesn’t work because it was on a test server, but this link for 0767886739 works and gives back some XML.
Nothing’s been done on this since, but if you want to help, you know where to download the source code and start hacking …