skel.js is a lightweight frontend framework for building responsive sites and apps. It has no dependencies and will play nice with (most) other libraries and frameworks.

HTML5 Up! released under the Creative Commons Attribution license some very good-looking, responsive and free HTML5-CSS3 templates.

Easy responsive tabs – is a lightweight jQuery plugin which optimizes normal horizontal or vertical tabs to accordion on multi devices like: web, tablets, Mobile (IPad & IPhone).

jQuery Transit is a plugin that help you do super-smoot CSS transformations and transitions in jQuery.