JavaScript

Favorites

Helper

String to Number conversion table
Polyfill: Js Compatibility table + Polyfill
FormatJS: Internationalize Client & Server side
Listeners: Usefull listeners

React

Awesome-Components: Top Components
Blueprint: UI Toolkit

Vue

Element: UI Toolkit
Vuestic-admin: UI

Front-End

React: Flux Model
UpUp: Offline manager

Tools

Package Manager

PackageQuality: Check the global quality of an npm package

Test

Coveralls: Code test coverage analysis (Free)
TestCafe: all front task test runner

Style guide

Idiomatic
[Brands-Styleguide] : Style guide examples from various Brands

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Layzr: Lazy image loader
FreeSSL: Free SSL Certificate
ORY: Interactive Website builder

HTML

Template

Freebie: Single page responsive Bootstrap theme

CSS

Sanitize: Consistent starter style
AniCollection: List of CSS animations
Animista: List of CSS animations (cooler).

CheatSheet

Other

Big-O Algorithm: Big-O Algorithm Sheet
HTML5 Boiler Plate: Project Starter kit
Free-for-dev: List of free tools for dev
GitIgnore: git .ignore helper
Design-Resources: Design resources
Awesome-Design: Awesome design