With the obvious lack of screen real estate on mobile devices, despite recent increases, creating user friendly navigation menus is pretty important. Personally, I find simple drop-down menus to be a little clunky and can cause problems if the user has to scroll. I much prefer menus that fold out from the side with their own scrollable area.
This article will run through a few of jQuery scripts that I have found which assist in creating slide-in navigation menus that you can use on any device including mobile/cell phones. Whether you are creating an adaptive or responsive website or just a mobile site, these may work for you.