WebKit, V8, and Microsoft Edge have each gotten a little closer to supporting the full ES2015 (ES6) spec and beyond. WebKit was the first browser engine to reach 100% on the Kangax Compatibility Table while V8 and Edge each added more features to their implementations.
The Node.js Foundation has released Node.js v 4.0, combining Node.js and io.js into one single codebase. The all-new Node.js contains many new ES6 features enabled by default, as well as V8 v4.5, with accompanying new features.
A team of former Opera developers along with their ex-CEO Jon von Tetzchner have created a new browser called Vivaldi.
The io.js team has released version 1.0 -- but the versioning does not suggest the platform is "production ready." Despite overtaking Node, io.js clarifies the release in its own FAQ: "The choice to release as 1.0.x was not to signify that io.js should be considered production-ready, but because it was a significant enough release from Node.js to warrant a major version increment," it says.
The recently released Dart 1.1 comes with better performance for the dart2js compiler and improves support for the server-side development. The language itself is pretty much on hold.
Opera will release new versions of their browser for mobile and desktop based on WebKit. They are also going to integrate Chromium.