Tactical multiplayer shooter Insurgency: Sandstorm, one of the hidden gems of 2019, is free to play from now until next Wednesday, April 1.
You can grab it from its Steam page—if you're enjoying it, then you can buy it at half price (£12.99/$14.99) for the duration of the free-to-play period.
It's a fairly realistic squad-based shooter that's constantly adding new guns, maps, and modes, and the tug-of-war style PvP Frontline mode is a standout. The latest update, 1.6, added full mod support, and all mod downloads are handled within the game client. The update also added a new map, Power Plant, and made all maps compatible with Domination mode, which was added in 1.5.
If you need more of a recommendation, check out Phil's 85/100 Insurgency review. He called it "fresh and innovative", and thought its focus on realism was able to "convey the ugliness and horror of modern military combat, without the need for overwrought scripted sequences".