What are some alternatives?
When comparing Unity and PlayN, you can also consider the following products
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Pygame - Pygame is a set of Python modules designed for writing games.
Unreal Engine - Unreal Engine 4 is a suite of integrated tools for game developers to design and build games, simulations, and visualizations.
Castle Game Engine - An open-source 3D/2D game engine for ObjectPascal.
GameMaker - "With GameMaker, I went from no programming knowledge to full-time developer in 2 years!" Seth Coster - Butterscotch Shenanigans - Crashlands. "GMS 2 is absolutely amazing and I cannot get over how great this software is to use".
CryENGINE - The most powerful game development platform is now available to everyone. Full engine source code.