Peter Merholz neatly summarizes the excitement around ethnoclassification, a.k.a. folksonomies, social tagging, etc. His contribution to the conversation is a call for system designers (del.icio.us, Flickr) to figure out a way to pave over the agreed upon and highly trafficked tags.
To add my 2 cents, as far as I know you can tag urls (del.icio.us), photos (Flickr), email (GMail), webfeeds (Rojo) and, slowly, blog posts. Someone needs to fix up the music players so people can start tagging tracks and playlists. Granted, this can be reduced to tagging urls, but the tags can actually be carried around in the audio files and playlists. Individual tracks might be too fine grained and too much work to be worthwhile, but tagged playlists would be fun!!