I just found a very interesting plugin for Movable Type: TagSupplementals. As the name says, the plugin provides extra features for working with tags.
I am using the plugin on El Canasto to create a Related Entries list on each individual entry page. The related entries feature of this plugin has two weighing modes: constant gives each tag the same weight and simply sums up the number of tags the entries have in common, while idf gives each tag a weight of
freq is the number of entries tagged with that tag.
Other features of the plugin, which I currently don't use, are: a tag count, related tags, a listing of all tags used in an archive, tag last update date, etc.