The Mobilize plugin uses CSS3 only to generate a mobile menu from an existing WordPress menu and displays it for resolutions lower than 720px width (customizable). The menu is assigned via Appearance -> Menus section and can be a new menu or an existing one (usually a duplicate of the main/primary menu).
Mobilize menu accepts an unlimited number of menu levels, although not more than 3 levels are recommended. The menu slides in from the left and it is called by the top link.
Mobilize works with any theme, doesn’t use any JS code and it depends on FontAwesome (included).
The backend options are no-nonsense, simple and clear.