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/

#gamedev

Godot Engine - Free and open source 2D and 3D game engine

Godot provides a huge set of common tools, so you can just focus on making your game without reinventing the wheel.

Godot Engine
@djlink Don't sleep on bevy! - https://bevyengine.org/
Bevy Engine

Bevy is a refreshingly simple data-driven game engine built in Rust. It is free and open-source forever!