The Collapsible library automatically collapsible the nav items into a dropdown menu with a hamburger toggle button, when there’s not enough screen space.
A Responsive Header Navigation that collapses the horizontal navbar into a dropdown menu by altering the flex-direction property on small screens.
Yet another responsive, cross-platform navigation system that converts the horizontal menu into an off-screen hamburger navigation on the mobile device.
Bungie Bar is an Apple.com inspired responsive navigation that collapses the long horizontal nav into a scrollable menu with left/right navigation arrows on mobile devices.
JMenu is a pure CSS responsive menu which automatically collapses the regular drop-down navbar into a mobile-friendly sliding menu with or without the toggle button.