A couple of interesting tagging (folksonomies, not graffiti!) projects from the arts/museum world:

Steve Museum
Steve Museum is a largescale tagging project which is supported by an ILMS grant. Partners include the Met and the Guggenheim.

Article about the Steve Museum project and tagging:

Steve Musuem (tagging also links to research, working papers, etc.)

development site:

The Cleveland Museum of Art (a Steve Museum partner) has a “Help Others Find Me” button, which allows users to enter tags.

Here is a link to a de Kooning work from the museum: