Mental Omega is a popular mod for the game Command & Conquer: Red Alert 2 and Yuri’s Revenge. It is a free fan-made expansion pack that adds new campaigns, structures, units, maps, and multiplayer modes. It is set in an alternate timeline and includes a variety of new factions, weapons, and technologies. It also includes a unique story line and original music. The game is designed to be a challenge for experienced players and includes a variety of difficult missions. It also has a thriving online community and offers a variety of tournaments and competitions. Mental Omega is one of the most popular mods for Command & Conquer and is a must-have for any fan of the series.
Discontinued Dune II is suspended because Westwood shutdown here is more info: https://www.gamespot.com/articles/ea-consolidates-studios-closes-westwood/1100-2909852/
OpenRA
Command and Conquer
Supreme Commander
Dune Legacy
Dune
Absolutely free open source engine for the original C&C series titles.