Armory 3d engine has recently gone free and open source with the permissive zlib license. It's very much in development and has a small team behind it, but it also has some features that may be interesting: for example, close integration with Blender and deployment to multiple target platforms. The latter is possible because it's based on the Haxe programming language.
https://armory3d.org/
I came across it recently in connection with Blender. Someone suggested it as an alternative to Blender Game Engine which is now officially discontinued in Blender 2.8. I have never used BGE, but Armory looked interesting enough to share with you folks on its own merits.
If you use Blender, it might already do something useful for you, maybe as a rapid prototyping tool using modern real-time rendering techniques. If you'd just like to have a peek at what it can do, they have demo videos on their Youtube channel, linked from the above site.
(Blender 2.8 is in early alpha ATM. Armory is not going to be a core part of it.)