Tether
A positioning engine to make overlays, tooltips and dropdowns better #hubspot-open-source
Mediaelement
HTML5 <audio> or <video> player with Flash and Silverlight shims that mimics the HTML5 MediaElement API, enabling a consistent UI in all browsers.
jQuery Pjax
pushState + ajax = pjax
Nprogress
For slim progress bars like on YouTube, Medium, etc
FileSaver.js
An HTML5 saveAs() FileSaver implementation
Toastr
Simple javascript toast notifications
Angular Strap
AngularJS 1.2+ native directives for Bootstrap 3.
Remark
A simple, in-browser, markdown-driven slideshow tool.
Popmotion
A 12kb Javascript motion engine. Use for animation, physics and input tracking.
jQuery Mobile
jQuery Mobile Framework
Magnific Popup
Light and responsive lightbox plugin with focus on performance.
Paper.js
The Swiss Army Knife of Vector Graphics Scripting – Scriptographer ported to JavaScript and the browser, using HTML5 Canvas. Created by @juerglehni & @jonathanpuckey
Sinon.js
Test spies, stubs and mocks for JavaScript.
Loaders.css
Delightful, performance-focused pure css loading animations.
Is.js
Micro check library
Browser Sync
Keep multiple browsers & devices in sync when building websites. http://browsersync.io