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I wrote an entry on yufind the other day, noting its use of xISBN to cluster editions in results. yufind is an implementation of VuFind. Here is another example of the clustering: Villanova.
As Till Kinstler points out in a comment, and as William Denton notes in a discussion of the entry on his FRBR blog, this is a feature supported in the standard VuFind distribution.
My colleagues Karen Coombs and Xiaoming Liu sent me some further information. Apparently, the VuFind developers are looking at potential integration also of the Worldcat API and Worldcat Identities.
A companions service to xISBN is xOCLCNUM. This would give better coverage (the ISBN only applies to recent items) but I do not have a good sense of how widely available an OCLC number would be in records of catalogs run by VuFind.