The getID3() Drupal module facilitates the installation and management of the getID3() PHP library, used to extract useful information from MP3s and other multimedia file formats.
The form_wizard module provides API functions that allow developers to easily create wizards using the improvements and new features of Form API in Drupal 6.
This module defines the "hierarchical_select" form element, which is a greatly enhanced way for letting the user select items in a hierarchy.
Hierarchical Select has the ability to save the entire lineage of a selection or only the "deepest" selection. You can configure it to force the user to make a selection as deep as possible in the tree, or allow the user to select an item anywhere in the tree. Levels can be labeled, you can configure limit the number of items that can be selected, configure a title for the dropbox, choose a site-wide animation delay, and so on. You can even create new items and levels through Hierarchical Select!
For a good overview of what Hierarchical Select can do, look at this demo!
This module provides an API for controlling the visibility of arbitrary elements, similar to block visibility settings in Drupal Core. The 6.x version of the module also provides form creation helper functions.