- ES6 Katas - Small, byte-size exercises to discover most aspects of ES6 features by doing interactive, online exercises. Pretty awesome.
- Learn ES2015 - A great part of Babel's website that takes you through examples of all supported ES6+ features
- ES6-Features.org - Nice ES6 / ES5 comparisons of many ES6 language features
- ES6 Features - A single-page tour of code examples for just about every ES6 feature, by Luke Hoban
- ES6 In Depth - A great series of articles on Mozilla Developer Network (MDN); also available in French through great translation efforts.
- Understanding ES6 - Excellent free book by Nicholas C. Zakas, covers everything.
- Exploring ES6 - Amazing free book by Dr. Axel Rauschmayer that goes into all the nitty-gritty with great example use cases.
- ES6… and beyond! - The latest, in-production chapter of Kyle Simpson's excellent You Don't Know JS series.
What about http://exploringjs.com/ Exploring ES6: Upgrade to the next version of JavaScript, by Dr. Axel Rauschmayer