flexMenu is a jQuery plugin for creating responsive menu bars. When there is only space to display some of the items in the menu, the rest of the items collapse into a "more" drop-down.
I wrote an article for Webdesigntuts+ on using this tool to build flexible navigation for responsive websites.
The source for this plugin is available on GitHub. I've been thrilled to see other developers contribute feedback, tweaks, and improvements. Collaborating with complete strangers on open source code has been exciting and inspiring.