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.

UPDATE:

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