Brief and interesting post @ RSS4Lib about projects to tag various library catalogs, and the relevance of lesser used tags. No mention of PennTags, but there is a brief mention of MTags from the University of Michigan.
The real value of tagging is like that of a library: it’s the collections, the constructed universe of things that someone (a librarian, a subject expert, a user) brought together. While my tags may prove of no value to anyone else in finding a particular item, the mass of items I’ve used that idiosyncratic tag on may very well guide a future user in resource discovery.
I couldn’t have said it better myself.