I am not presenting the actual approach here. I created this blog entry just to point to this wonderful tutorial in Drupal’s official theming guide for this purpose, which describes all the steps in a clear and concise way.

I came across it while creating a drop-down menu for one of my client’s Drupal based sites, and the above tutorial helped me complete the menu in inside a couple of hours. However, there is a minor bug in the original code posted there for page.tpl.php towards the end of the tutorial (which I have edited on the tutorial itself).

If you still have problems getting the drop-down menu to work, you can discuss it below. However, take care that you would need a free or paid client-side dropdown menu javascript, and you would need to modify your theme css accordingly to specify styles for drop-down menu items. This css would mostly be dictated by the dro-down script you choose.