DOMAssistant : The modular lightweight JavaScript library, with CSS selectors, event handling, AJAX and complete Unicode support
Workbox : JavaScript Libraries for adding offline support to web apps.
Styletron : Styletron is a universal CSS-in-JS engine built from the ground up for high-performance.
jQuery Masonry : A dynamic layout plugin for jQuery
The flip-side of CSS floats
rsvp.js : A lightweight library that provides tools for organizing asynchronous code
Rainbow is a code syntax highlighting library written in Javascript.
It was designed to be lightweight (1.4kb), easy to use, and extendable.
It is completely themable via CSS.