Dropdown menus in Drupal the way I do it. By using the menu_block I am more flexible in how to display the menu. By using block_class I can reuse the css code without having to worry about the id drupal generates for the block.
Step 1. Install menu_block and block_class.
Step 2. Goto /admin/blocks and create a menu_block item with expanded children, a block classname "menublockmainmenu" and place where you like.
Step 3. Add css code.