OpenRA

OpenRA is a free, libre real-time strategy game engine that reimagines the early Westwood classics. It recreates and modernizes Command & Conquer: Red Alert, Command & Conquer: Tiberian Dawn, and Dune 2000, so the familiar RTS gameplay runs natively on current platforms with quality-of-life improvements instead of the original DOS-era releases.

Those three games ship as the engine's distributed mods, complete with online multiplayer and skirmishes against the AI. Maps can be reshaped with custom rules, missions scripted through a Lua API, and entirely new classic-style RTS games built using the OpenRA Mod SDK.

Written in C# with SDL and OpenGL, OpenRA runs on Windows, Linux, BSD, and Mac OS X, and dedicated game servers can be self-hosted. It is licensed under GPL-3 or later.