There are several modules and approaches for creating different types of dropdowns for Drupal sites, usually for the primary links. A comparison and explanation of these would be very helpful for new themers.

Deliverable:

Create a handbook page that explains what a dropdown menu is, and compares the following modules/approaches -- ease of use, how well supported they are, the features included, etc.

http://drupal.org/project/nice_menus
http://drupal.org/project/megamenu
http://drupal.org/project/superfish

You can also code dropdown menus into your theme, using CSS/JS. Or use a (base) theme that includes them out of the box, eg.
http://drupal.org/project/corolla
http://drupal.org/project/fusion
http://drupal.org/project/pixture_reloaded