Transpilers

• Traceur compiler - ES6 features > ES5/3. Includes classes, generators, promises, destructuring patterns, default parameters & more.
• es6ify - Traceur compiler wrapped as a Browserify v2 transform
• es6-transpiler - ES6 > ES5. Includes classes, destructuring, default parameters, spread
• Square's es6-module-transpiler - ES6 modules to AMD or CJS
• Square's es6-arrow-function - ES6 arrow functions to ES5
• Facebook's regenerator - transform ES6 yield/generator functions to ES5
• Facebook's jstransform - A simple utility for pluggable JS syntax transforms. Comes with a small set of ES6 -> ES5 transforms
• defs - ES6 block-scoped const and let variables to ES3 vars
• es6_module_transpiler-rails - ES6 Modules in the Rails Asset Pipeline
• Some Sweet.js macros that compile from ES6 to ES5

Build-time transpilation

Grunt Tasks

• Traceur: grunt-traceur ES6 > ES6 transpilation, grunt-traceur-build
• ES6 Module Transpiler: grunt-es6-module-transpiler
• Regenerator: grunt-regenerator - ES6 generator functions to ES5
• grunt-microlib - tools for libs using ES6 module transpiler (sample Gruntfile)
• grunt-defs - ES6 block scoped const and let variables, to ES3
• es6-transpiler: grunt-es6-transpiler - ES6 → ES5

Gulp Plugins

• Traceur: gulp-traceur
• Regenerator: gulp-regenerator
• ES6 Module Transpiler: gulp-es6-module-transpiler
• es6-transpiler: gulp-es6-transpiler - ES6 → ES5

Module Loaders

• ES6 Module Loader polyfill (compat with latest spec and Traceur)
• js-loaders - Mozilla's spec-compliant loader prototype
• JSPM - ES6, AMD, CJS module loading/package management
• Module Loader for webpack
• beck.js - toolkit for ES6 Module Loader pipelines, shim for legacy environments

Code generation

• generator-traceur - Yeoman generator for Traceur apps
• grunt-init-es6 - scaffold node modules with unit tests, authored in ES6
• Loom generators with ES6 ember modules
• Brunch plugin for ES6 module transpilation

Polyfills

• es6-shim - almost all new ES6 methods — from Map, Set, String, Array, Object, Object.is and more.
• WeakMap, Map, Set, HashMap - ES6 Collections
• Polymer's WeakMap shim
• String.prototype.startsWith
• String.prototype.endsWith
• String.prototype.at
• String.prototype.repeat
• String.prototype.contains
• String.prototype.codePointAt
• String.fromCodePoint
• Array.prototype.find
• Array.prototype.findIndex
• Array.from
• Array.of
• Object.assign
• es6-promise - polyfill for Promises matching the ES6 API
• ES6 Map Shim - destructive shim that follows the latest specification as closely as possible.
• ECMAScript 6 polyfill
• Function.create
• ES6 shim
• ES6 Symbol polyfill
• ES6 Map, Set, WeakMap
• harmony-reflect - ES6 reflection module (contains the Proxy API)

Editors

• ES6 syntax highlighting for Sublime Text and TextMate
• DocPad plugin for Traceur

Other

• ES.next showcase - real-world usage examples of ES6 features
• looper - static analysis tools for ES6
• es6-module-packager
• es6-import-validate and grunt-es6-import-validate - validate matching named/default import statements in ES6 modules.
• let-er - transpiles let-block block-scoping (not accepted into ES6) into either ES3 or ES6
• Esprima Harmony branch - Experimental branch of the Esprima parser which can parse ES6 features.
• Recast - Esprima-based JavaScript syntax tree transformer, conservative pretty-printer, and automatic source map generator. Used by several of the transpilers listed above, including regenerator and es6-arrow-function.