BuddyPress.org

Well, with alpha 2. Site admins will create member, group and blog categories. Then members can choose one or more cats they fit. Same for group and blog admins. It’s the site admin’s site. They get to determine the categories.

Of course cats can be added or deleted as needed by the site admin. The site admin sets everything up and people pick and choose appropriately where they fit in. The member, group and blog tags extend and enhance this by allowing free form additions to the categorization process. Then either or both the cats/tags could be used to discover people, groups or blogs.

Since the categories aren’t as fluid as the tags, you could allow users to categorize themselves during signup. People can use categories to get a broad idea of the site content and then narrow it down with tags. Things like that.