Today it’s Operation Flashpoint: Cold War Crisis a 2001 tactical shooter and battlefield simulator video game developed by Bohemia Interactive Studio and published by Codemasters for Microsoft Windows (PC). In 2011, Bohemia Interactive re-released the game under the title ARMA: Cold War Assault.
Steam have a sale currently on all ARMA games as part of their Midweek Madness. You can now get all for 50% off.
The ARMA X: Anniversary Edition is the one to get if you want to get all the games in the franchise. It also includes ARMA: Gold Edition for free. However if you want to get certain games then you will still be saving money overall.