Toastr
Simple javascript toast notifications
Tether
A positioning engine to make overlays, tooltips and dropdowns better #hubspot-open-source
FileSaver.js
An HTML5 saveAs() FileSaver implementation
Onsen Ui
Custom Elements-Based HTML5 UI Framework for Building Your Mobile Front End
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
Remark
A simple, in-browser, markdown-driven slideshow tool.
Nock
HTTP mocking and expectations library
Showdown
JavaScript port of Markdown
Angular Strap
AngularJS 1.2+ native directives for Bootstrap 3.
Loaders.css
Delightful, performance-focused pure css loading animations.
Browser Sync
Keep multiple browsers & devices in sync when building websites. http://browsersync.io
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
jQuery Mobile
jQuery Mobile Framework