Tim Appnel’s written some great plugins for Movable Type and now he’s got a slick tag plugin called Tags.App that converts MT’s hierarchical categories into tags:

The basis of Tags.App was originally developed for The O’Reilly Radar weblog … back in March. It essentially brought tagging to MT. It provides the means to dynamically navigate tags and see tag intersections, in addition to display tag data as lists and clouds. In the last release I added the ability to view tags across multiple weblogs and create tag clouds from other data sources.

Note that Tags.App is a commercial product. Tim also offers a free scaled-down version of the plugin called mt-tagslite.

Compare with the Tags plugin provided as part of the “official” Power Tools collection, which uses the keywords field.

Another tag option for MT is Tagwire, which differentiates itself from Tags.app and Tagslite by its support of multilingual tagging and the use of the keywords field, leaving categories separate. According to its creator, it can “[lead] to performance degradation especially for rebuilding, but Tagwire employs PluginData and Request Cache effectively and achieves enough speed.” Also: “Though Tagwire generates no static tag archives, it couples with Tim Appnel’s MT-XSearch and supports Dynamic Tag Archiving.”