Casper JS
Navigation scripting & testing utility for PhantomJS and SlimerJS
Tether
A positioning engine to make overlays, tooltips and dropdowns better #hubspot-open-source
Mjml
MJML: the only framework that makes responsive-email easy
Page.js
Micro client-side router inspired by the Express router (~1200 bytes)
Sigma
An open-source lightweight JavaScript graph drawing library
Scroll Magic
The jQuery plugin for magical scroll interactions.
Loaders.css
Delightful, performance-focused pure css loading animations.
Pace
Automatically add a progress bar to your site. #hubspot-open-source
Raphael
JavaScript Vector Library
Vivus
JavaScript library to make drawing animation on SVG
Sinon.js
Test spies, stubs and mocks for JavaScript.
Easel JS
The Easel Javascript library provides a full, hierarchical display list, a core interaction model, and helper classes to make working with the HTML5 Canvas element much easier.
List
Do you want a 3 KB (gzipped&minified) cross-browser native JavaScript that makes your plain HTML lists and tables super flexible, searchable, sortable and filterable? Yeah! Do you also want the possibility to add, edit and remove items by dead simple templating? Hell yeah!
Svg.js
A lightweight library for manipulating and animating SVG
jQuery Mobile
jQuery Mobile Framework
Zxcvbn
zxcvbn, named after a crappy password, is a JavaScript password strength estimation library. Use it to implement a custom strength bar on a signup form near you!