Drupal’s Nice Menus Module provides a highly customizable and nice-looking dropdown menu for Drupal websites. To allow maximum customizability, it includes a CSS file and a javascript file where you can try out your own recipes. The following steps should help you make nice looking drop down menus for your Drupal website using the Nice Menus module:

Step1: Download Nice Menus module from http://drupal.org/project/nice_menus
Step2: Extract the downloaded tar file to Drupal’s sites/all/modules directory.
Step3: Enable Nice Menus module from Drupal’s modules listing page at admin/build/modules (Menu: Administer > Site building > Modules).
Step4: Create sub-menus for the primary menu through the menu administration interface.
Step5: Configure the Nice Menu block from admin/build/block (Administer >Site building >Blocks). “Nice menu 1 (Nice menu)” and “Nice menu 2 (Nice menu)” are in the disabled section.
Step6: Click the 'configure' link against each to edit the block’s settings.
Step7: Select

 ‘Menu Parent’ -> primary links
 ‘Menu Style -> right or left or down

Step8: Save block configuration
Step9: Clear cache.

The menu should now appear where you wanted it to.

To further customize this, edit the included CSS file.