A couple free & open-source Engines and game frameworks you can use today as alternatives. Feel free to comment with more.
Godot - https://godotengine.org
MonoGame - https://www.monogame.net/
FNA - https://fna-xna.github.io/
SDL - https://www.libsdl.org/
SFML - https://www.sfml-dev.org/
Adventure Studio - https://www.adventuregamestudio.co.uk/
bgfx - https://github.com/bkaradzic/bgfx
armory3d - https://armory3d.org/
ogre3d - https://www.ogre3d.org/
Renpy - https://www.renpy.org/
@djlink some more obscure ones:
monogame (Microsoft xna clone, notable for powering the 360 era of indie games):
https://www.monogame.net/
iD tech 4: (gpl v3, must release your source)
https://github.com/iodoom/iod3
Torque (MIT, notably used in BeamNG): https://torque3d.org/
O3de (cryengine/lumberyard lineage but mit/apache, not used a lot yet):
https://o3de.org/
monogame (Microsoft xna clone, notable for powering the 360 era of indie games):
https://www.monogame.net/
iD tech 4: (gpl v3, must release your source)
https://github.com/iodoom/iod3
Torque (MIT, notably used in BeamNG): https://torque3d.org/
O3de (cryengine/lumberyard lineage but mit/apache, not used a lot yet):
https://o3de.org/
@xianc78 @djlink it's not showing up on my end if he did and even then current .net is mit licensed. It's using a more Java style business model.
