Vertical Tabs module by Dmitri Gaskin is a Drupal module-ification of prototype work I did for the usability of Drupal's node forms, and was inspired partly by ideas in the discussion and design process of Views 2 UI – which was all part of my Season of Usability project. I am using vertical tabs on 4 Drupal 6 websites I maintain or am developing and am extremely happy with how it turned out. While I would have like to have seen this committed to Drupal core, there was not enough time in the project for that, and indeed the module better serves for prototyping and testing purposes to first ensure that it is a change we want to make to core. Download the dev release of Vertical Tabs to try it out. The only obvious bug is that it is note color.module-enabled.