For those of you who blog, I think you'll find this cognitive analysis of tagging by Rashmi Sinha very interesting. Here is an excerpt of her post (I highly recommend reading her whole post if this area interests you):

"At the start, let me confess that I struggled with this topic. From my first encounter with tagging (on systems such as del.icio.us & flickr), I could feel how easy it was to tag. But it took me a while to understand the cognitive processes at work. What follows is Rashmi's theory of tagging - my hypothesis about the cognitive process that kicks into place when we tag an item, and how this differs than the process of categorizing. In doing so, my hope is to explain the increasing popularity of tagging, and offer some ideas regarding the design of tagging / categorization systems.

My ideas are mostly based on my observations about how people tag and relating it to on academic research in cognitive psychology and anthropology. This is a first version, which I expect to revise as I learn more. Feedback is very welcome.

The rapid growth of tagging in the last year is testament to how easy and enjoyable people find the tagging process. The question is how to explain it at the cognitive level. In search for a cognitive explanation of tagging, I went back to my dusty cognitive psychology textbooks. This is what I learnt..."