Alternatives to QuakeSpasm

First person shooter engine

2D and 3D game engine

Real-time strategy game engine for Westwood games

Open-source engine for Heroes of Might & Magic III

C# scripting capable version of Godot game engine

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

Enhanced source port of Doom, Heretic, and Hexen

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

Build engine port backed by GZDoom tech

Game engine for development of desktop, mobile and web games

Complete development tool for making 2D games

Lua-based game engine