Voyeur.js
Voyeur is a tiny (1.2kb) Javascript library that lets you traverse and manipulate the DOM the way it should have been.
Jquery Waypoints
Waypoints is a small jQuery plugin that makes it easy to execute a function whenever you scroll to an element.
Tether
A positioning engine to make overlays, tooltips and dropdowns better #hubspot-open-source
Scrob Master
Master your scroll events.
Gliojs
Exit popup easely. Detects if the mouse of a user leaves the viewport borders of your website
Packery
bin-packing layout library
Persist State
jQuery Ui widget that will transparently save the state of checkboxes, inputs, selects, etc in localStorage, restoring these on the user's next visit to the page.
Fragment.js
A tiny tool for easily loading html fragments and templates
Eqcss
EQCSS is a plugin that introduces Element Queries, Scoped CSS, a Parent selector, and responsive JavaScript to all browsers IE8 and up
Porthole
A proxy to safely communicate to cross-domain iframes in javascript
jQuery.lazy
A lightweight, fast, feature-rich, powerful and highly configurable delayed content, image and background lazy loading plugin for jQuery & Zepto.
Double
jQuery plugin for hiring doubles
jScrollPane
Pretty, customisable, cross browser replacement scrollbars
Viewport
Gets the dimensions of the Viewport and beyond.
Egjs Infinitegrid
A module used to arrange card elements including content infinitely on a grid layout.
jQuery Evergreen
Small & fast DOM and event library for modern browsers.
Node List.js
NodeList implementation/library - Use the Native DOM APIs as easily as jQuery