Old-school free RTS. Looks amazing. Cross platform as well.

Here's a ridiculously fun open-source RTS game that reminds me a bit of Warcraft 2. Has single-player campaigns as well as multi-player.

MegaGlest is an entertaining free and open-source cross-platform 3D real-time strategy (RTS) game, where you control the armies of one of seven different factions: Tech, Magic, Egypt, Indians, Norsemen, Persian or Romans.

An open-source engine reimplementation of the classic Diablo (I) game. Requires diabdat.mpq from the original game in order to play.

Here's an open-source clone of Civilization II. Freeciv is a Free and Open Source empire-building strategy game inspired by the history of human civilization.

Here's a neat open-source (slow-build) RTS that's similar to Settlers II. It focuses on developing your settlement, yet also has combat. Widelands has a single-player campaign as well as multiplayer.

Another free, open-source RTS based on the Spring engine. Available through Steam and itch.io.

Futuristic open-source RTS. Available through Steam and itch.io.

Play a classic DOS game online.