thingISBN
Tim Spalding at LibraryThing introduces thingISBN, an equivalent OCLC’s xISBN. If you give it an ISBN it will return a list of the ISBNs of all of the other manifestations it knows of for the same work. The difference is that where OCLC has smart people making complicated algorithms that run on WorldCat, LibraryThing uses the decisions made by all its users when they group things together into works. Don’t miss Spalding’s blog entry about this.