Similarly to the ratings and comments, I am developing a tag system allowing artwork to be categorised.
Unlike the more traditional forms of categorisation like folders, tags allow the same bit of information to be stored under several categories, without duplicating the information.
Each individual piece of artwork can be tagged by every user on the system, with popular tags rising to the top.
You can look at a preview of a tag-based gallery page at: http://portfolio.cgsociety.org/gallery/