Plugin

Formstone
A collection of thoughtfully crafted front-end components focused on simple implementation and easy customization.
Unison.js
Unison.js is a small plugin (1.2kb minified) that allows you to declare named breakpoints in one place and automatically sync them across your javascript and markup.
Crowdspell
Crowdspell is a free and small script that allows the reader of an article to correct a typo very easily and quickly.
easystar.js
asynchronous pathfinding in javascript.
tether
A client-side library to make absolutely positioned elements attach to elements in the page efficiently.
Chardin.js
Simple overlay instructions for your apps.
InstantClick
InstantClick is a JavaScript library that dramatically speeds up your website, making navigation effectively instant in most cases.
Isomer
Isomer is available for free under the MIT License, but you can also pay for it.
rainyday.js
The idea behind rainyday.js is to create a JavaScript library that makes use of the HTML5 canvas to render an animation of raindrops falling on a glass surface. Simple enough, right?
Trianglify
Trianglify is a javascript library for generating colorful triangle meshes that can be used as SVG images and CSS backgrounds.
simpleWeather
Pretty much the best plugin in the world.
passy
passy is a jQuery plugin to rate and generate passwords.
UIBox
Curated HTML, CSS, JS UI Component Library.
Olly.js
Translate URLs into Rich Media.
fast.js
Faster user-land reimplementations for several common builtin native JavaScript functions.
fastclick
Polyfill to remove click delays on browsers with touch UIs.
Spinners
Angular Spinners at http://stanleygu.com/spinners/
Intro.js
A better way for new feature introduction and step-by-step users guide for your website and project.