Feedvis.com generates a tag cloud and lists word frequencies from an opml export of your rss feeds. Below, for example, is a section of the feeds in the renowned Knowbodies/Netvibes metagator. On the left, the words and their frequencies, and on the right the blogs issuing the feeds. You can analyse all feeds collectively (as I've done below), or analyze one particular feed. If you're interested in tag clouds (personally I think they look nice, but don't understand what they're good for) Feedvis.com is a neat plaything. Regarding the permanence of the site, the person who developed it says "Not very, it's mostly a project to help me learn programming."