Yahoo News Tag Soup takes Yahoo News stories, extracts key concepts from article headlines and summaries, then automatically tags them into different categories. The results is a list of tags shown in "tag cloud" style, where the most popular topics show up bigger. Click on a category of interest, and you'll see all the related stories for that topic.
Pretty cool -- but why does the default have to always be alphabetical with these things? Why not show clouds ordered with the most popular categories coming first?
By the way, technically this isn't tagging in the popular sense, where various members of a community label objects according to whatever syntax they want. This is clustering, where a group of documents is automatically organized into categories. But it looks and feels like tagging, so those who like to explore tag categories will feel right at home.
FYI, Yahoo News has for years had similar functionality in the apparently non-cool Full Coverage category system. Check out Yahoo Full Coverage, and you'll see stories topics placed into categories (ie, tags).