When I first started curating the Pattern Library, I put "tags" near the top of my list of user interaction patterns to investigate. By that time, Yahoo! had already acquired several pioneers in the tagging realm, Flickr and Delicious, and there were some subtle distinctions in how they implemented the experience.
We got down in the weeds on these and did a lot of research, ultimately settled on offering high-level guidance, and finished the patterns in the course of writing the social patterns book, where we filed tagging under the group of patterns known as Collecting, under Social Objects.
Tagging and other forms of collecting are also an example of social design patterns that mimic game dynamics. Collecting objects is a core "easy fun" activity in many games, and similarly these extremely lightweight social interactions around gathering or tagging objects enable a form of self-interested behavior that creates aggregate value and potentially richer forms of engagement.
Our three new tagging patterns are Tag an Object, Find with Tags, and the somewhat controversial Tag Cloud, which some people view as an "anti-pattern." Drop by, check them out, and let us know if we can make them any better.
Curator, Design Pattern Library