Voyeur.js
Voyeur is a tiny (1.2kb) Javascript library that lets you traverse and manipulate the DOM the way it should have been.
Tire
Tire is a lightweight JavaScript library for modern browsers
Cortex
A javascript library for centrally managing data with React
Gliojs
Exit popup easely. Detects if the mouse of a user leaves the viewport borders of your website
jScrollPane
Pretty, customisable, cross browser replacement scrollbars
Responsive Comments
A client-side solution to conditional loading.
Interact.js
Drag and drop, resizing and multi-touch gestures with inertia and snapping for modern browsers (and also IE8+)
Resize Observer
Polyfills the ResizeObserver API.
Laconic
Sane DOM Generation
Zepto
Zepto.js is a minimalist JavaScript library for modern browsers, with a jQuery-compatible API
Minified.js
Replace jQuery or MooTools with a <4kB Library
Gator
Event delegation in Javascript
jQuery Evergreen
Small & fast DOM and event library for modern browsers.
Design Blocks
A set of 170+ Bootstrap based design blocks ready to be used to create clean modern websites.
Veinjs
Injects CSS into the document (it's a perfectly legal procedure)
Sizzle
A sizzlin' hot selector engine.
React Virtualized
React components for efficiently rendering large lists and tabular data