A new service from OCLC:
Just as xISBN allows you to find all related editions of a book by entering its ISBN, xOCLCnum does the same thing using OCLC number.
xOCLCnum is queried using a simple URL format, and returns an XML response with both related OCLCnums and related ISBNs (if any). It is designed to be easily built in to your library application, so you can expand queries, find all related editions, or do whatever creative thing you want to do.
Background: ISBNs have been assigned since 1970, to most but not all books published.OCLC numbers are assigned whenever a record is added to WorldCat, OCLC's global union catalog. These records cover a large portion of all books, old and new, held by any library in North America and, increasingly other regions worldwide.
So the coverage range of OCLC numbers is far greater than that of ISBNs: in WorldCat, for example, around 100 million OCLCnums compared to about 20 million ISBNs.
See website: http://xisbn.worldcat.org/xisbnadmin/xoclcnum/index.htm