Godot Engine just keeps on advancing in new and interesting ways. This free and open source game engine can now be run in a web browser - yes really.
Writing on the official blog, developer Fabio Alessandrelli mentioned that thanks to a sponsorship from Mozilla they've been able to make Godot Engine available as a HTML5 application. Currently, it needs either Firefox Nightly or a very recent Chromium based browser, due to the features it needs like Shared Array Buffer.
Amazing what you can do in a browser window now.