del.icio.us (I ♥ delicious) has added the ability to group tags (called bundles). You can’t actually create namespaces or heirarchies or anything like that, they’re more like labels for sets of tags. Still, it’s a nice way to break up the big list o’tags, and you can do some pseudo faceted classification now. See bundles in action or make your own bundles (if you use delicious that is) http://del.icio.us/settings/USERNAME/bundle.