Holder
Holder renders image placeholders entirely on the client side.
Grunt
Grunt: The JavaScript Task Runner
Boomerang
End user oriented web performance testing and beaconing
Rulers Guides.js
Creates Photoshop-like guides and rulers interface on a web page
Nock
HTTP mocking and expectations library
Jasmine
DOM-less simple JavaScript testing framework
Blaver
A JavaScript library built on top of the Faker.JS library. It generates massive amounts of fake data in the browser and node.js.
Nightwatch
UI automated testing framework based on node.js and selenium webdriver
Csscritic
A lightweight framework for regression testing of Cascading Style Sheets
Bluebird
Bluebird is a full featured promise library with unmatched performance.
Trace Kit
Attempts to create stack traces for unhandled JavaScript exceptions in all major browsers.
Angular Debaser
Just a better way to test AngularJS apps.
Casper JS
Navigation scripting & testing utility for PhantomJS and SlimerJS
Recoil
Recoil is an experimental state management library for React apps. It provides several capabilities that are difficult to achieve with React alone, while being compatible with the newest features of React.
Qunit
An easy-to-use JavaScript Unit Testing framework.
Fast Check
Property based testing framework for JavaScript (like QuickCheck) written in TypeScript
Browser Sync
Keep multiple browsers & devices in sync when building websites. http://browsersync.io