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

Platform for 3D content

Complete development tool for making 2D games

Lua-based game engine