Alternatives to Doomsday Engine

2D and 3D game engine

Real-time strategy game engine for Westwood games

Build engine port backed by GZDoom tech

C# scripting capable version of Godot game engine

First person shooter engine

Engine for iD software's Quake

Open-source engine for Heroes of Might & Magic III

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

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

Game engine for development of desktop, mobile and web games

Complete development tool for making 2D games

Lua-based game engine