Alternatives to Marathon

First-person shooter, third in a trilogy

First-person shooter, second in a trilogy

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

Arena-style first person shooter

2D and 3D game engine

GameStream client

Game launcher

First person shooter engine

Real-time strategy game engine for Westwood games

Online multiplayer game platform

C# scripting capable version of Godot game engine

Frontend for emulators, game engines and media players (OpenGL graphics API)