Social bookmarking is everywhere these days. If your're at home building the next amazing del.icio.us- or digg-clone it's important you plan the design of the database carefully. Philipp Keller shows a few variations on the post "Tags: Database schemas" and gets an enormous response in the comments.
Alex Hillman headlines his blog with the sentence "tagging isnt just for hoodlums anymore" and I couldn't agree more. Tags and social bookmarks are just everywhere and part of most Web2.0 services.
On my web site there's not that much content yet. But I feel I need to upgrade my categories soon and perhaps add a tag cloud.
Jason Kottke is having similar thoughts and writes an interesting post about how to calculate how much the popularity of the tags are accelerating. In this way you can produce a list of tags that represents what's been going on recently on the site. Sort of like the "hot tags" on Flickr.