CSS drop downs seem to be a logical choice. The advantages are plenty and the implementation simple.
Css driven drop-downs rely on modifying the visibility/display of items with the :hover CSS pseudo-class.
Here is a nice demo: CSS3 Dropdown Menu.
Unfortunately with the simplicity of just two states (hover, non-hover) you loose the ability to add fancy transitions.
A particular problem with css 3 drop downs is that the visibility of the dropdown is applied immediately on mouse over. This is a nuisance for users as their mouse might just be passing by.
A nice plugin introduced the concept of hover Intent, which addresses this problem.
Below is a simple script that progressively enhances a css driven navigation.