Your probably already aware of the OCLC top holdings lists. I was going to pass over a post at O’reilly on it but came across this:

Lorcan also says that they are planning to offer a web service to look up the library holding rank for any title.

I can see this being used in OpenWorldCat or similar offerings to give a little rank number similar to Amazon’s sales rank. It will be interesting to see how libraries use it in their sites/opacs.