Some people call it a "tag cloud", others gave it the name "folksonomies" - they're all the same thing, and in this posting from DanandSherree.com, they show us how to create a "weighted category list" much like the tag clouds seen on Flickr and del.icio.us.

Similar to the weighted category list, the weighted keyword list builds a list of keywords used on your site and uses varying font sizes to show how popular they are. This list might be more representative of what your site is about than the weighted category list is.

The code is very simple, and can be adapted to other things besides Movable Type with a quick change of resources that it pulls from - the logic is all there...