Absolutely recommend if you like Red Alert or RTS - OpenRA.

Turn-based strategy Battle for Wesnoth. It's old, I played it first 16 years ago, but I returned to it recently and it's OK.

Cataclysm: Dark Days Ahead I have not played, only saw reviews, but it has its fans.

Shattered Pixel Dungeon for some roguelike action.

0AD is another RTS, I've heard good things about it, but haven't played it myself yet.