First off (and you probably read it elsewhere before): The awesome Godot game engine got a really awesome 2.0 release a few weeks ago. It also got independent of it's original developers (who stay active in development), and the first larger commercial game with it was released on Steam just a few days ago.

Fresh of the press is a FOSS release of an pretty awesome competitor though:
The Atomic Game Engine was just released fully under the MIT license! Its render engine is a fork of the pretty nice Urho3D renderer, but it includes an cross-platform integrated development environment similar to Godot:
Looks nice, but where is the Linux binary release? :(
Dive into the source code here. Similar to Godot it features some neat platform support: Linux, Android, WebGL, Windows, iOS and OSX (resorted for significance ;) ); but unlike it you have much more and more common scripting languages at your disposal: JavaScript, TypeScript, C++, and C# scripting in the works. Especially the latter could be interesting if someone manages to make an Unity3D compatibility layer for migrating and open-sourcing Unity games...

For a nice overview, don't miss the GamesFromScratch video and introduction tutorial (from back in December 2015 when it was not yet MIT licensed):