It's been 20 years since Bohemia Interactive released Operation Flashpoint: Cold War Crisis, the tactical FPS that gave rise to the ARMA series, and was later renamed Arma: Cold War Assault...to celebrate you can grab ARMA: Cold War Assault free on Steam and GOG for the next 2 days...
https://www.gog.com/game/arma_cold_war_assault?pp=d965a463ac183af4828302b14522bb3371cec49f
https://www.gog.com/game/arma_cold_war_assault?pp=d965a463ac183af4828302b14522bb3371cec49f