Here's my first plugin for Habari.
Magic Tags allows you to use tags to help with formatting and organization. Any tag begining with "@" is not displayed in $post->tags_out. This allows you, for example to tag "asides" with "@aside" and set that up to display in a different format, without having an "asides" tag for the site.
The plugin also includes a function for displaying all of the tags used on the site, similarly filtered.
I've updated the site tags list to display the number of posts for each tag, and sort by that number. It also hides any tags that have a post count of zero. Download is in the same place.
Download it here