project page - http://drupal.org/project/superfish
what does it do? - Superfish integrates jQuery Superfish plugin with your Drupal menus. There is another module called Nice Menus that can be compared to Superfish. The developer said the things that separate Superfish from Nice Menus is the fact that it offers many more features but most importantly it does not have any extra markup - or anything of the sort. Meaning the Superfish takes care of everything.
check out these websites for additional information and demonstrations on the Superfish module