Developer New World Interactive's hardcore multiplayer tactical shooter Insurgency is currently free on Steam - and if you pick it up now, it'll remain in your library to play whenever you want.
Insurgency, if you're unfamiliar, originally released for PC in 2014 and was the fully fledged standalone successor to New World Interactive's popular Half-Life 2 mod Insurgency: Modern Infantry Combat. The series gained quite a following (and still has an active player base today), thanks its more realistic take on the classic first-person shooter formula.
In Insurgency, for instance, head shots are instantly fatal, and you don't have access to a mini-map, on-screen crosshairs, even an ammo or health counter - all of which makes for an experience that rewards tactical planning and carefully orchestrated team work.