Libgdx

Open-source, cross-platform 2D/3D/multiplayer game engine designed for everyone.

Java / JavaFX / Kotlin Game Library (Engine)

Open Source real-time strategy game engine for early Westwood games such as Command & Conquer: Red Alert written in C# using SDL and OpenGL.

A dead simple 2D game engine for Go

A Flutter based game engine.

Shattered Pixel Dungeon is an open-source traditional roguelike dungeon crawler with randomized levels and enemies, and hundreds of items to collect and use.