Jim Hinck at viaLibri has unveiled a really nifty new feature: Library Collection Search. Enter in author, title, keyword or a combination of those and then select from a list of ten national/international union catalogs or twelve major library OPACs. I just took this feature for a test drive, and found it worked excellently.
Searchable union catalogs: WorldCat, COPAC, KVK, CCPB, ICCU, ESTC, VD17, ISTC, STCN, artlibraries.
Searchable OPACs: LC, BL, Hollis, Yale, Huntington, Folger, Morgan, AAS, Getty, CCA, Chicago Art Institute, Kew. Jim notes that he's taking suggestions for additions.
I like it! Simple, fast, and useful - doesn't get better than that.