Simple Tree Menu creates a visually-simple menu inspired by the Mac OS X Finder tree.
It uses a straightforward HTML structure based on >ul< that is the same as the Son of Suckerfish drop-down menus, so it's easy to learn, and it degrades gracefully.
There's no complex options to pass in and your preferred style can be created in CSS. The source-code download includes CSS and PNG icons to create that that Apple Finder look, or you can nest other HTML content to create something like Google's calendar navigation.