Alternatives to ioquake3

2D and 3D game engine

Engine for iD software's Quake

Real-time strategy game engine for Westwood games

Open-source engine for Heroes of Might & Magic III

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

C# scripting capable version of Godot game engine

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

Enhanced source port of Doom, Heretic, and Hexen

Build engine port backed by GZDoom tech

Platform for 3D content

Game engine for development of desktop, mobile and web games

Complete development tool for making 2D games

Lua-based game engine