Alternatives to OpenRA

2D and 3D game engine

C# scripting capable version of Godot game engine

Open-source engine for Heroes of Might & Magic III

Open-source, cross-platform game engine designed to be used by everyone

Build engine port backed by GZDoom tech

Game engine for development of desktop, mobile and web games

First person shooter engine

Engine for iD software's Quake

Open-source continuation of Bungie's Marathon 2 game engine

Enhanced source port of Doom, Heretic, and Hexen

Platform for 3D content

Lua-based game engine