Chris Roberts

This plugin makes it easy to add a sophisticated jQuery powered drop-down menu to your WordPress theme.

Mensy

Enables easy creation of drop-down menus in your WordPress theme.

muhsin

This widget takes all the links and put them in a drop-down select instead of displaying them as links scattered all over the sidebar. 
Its meant to be very light weight and down to earth.

Drop Down Links

This widget takes all the links and put them in a drop-down select instead of displaying them as links scattered all over the sidebar.

Mensy vs Drop Down Links