For one month, starting tomorrow (April 19th to May 18th) all DCS world modules (except for campaigns) will be freely available for anyone and all modules will be 50% off.
DCS (Digital Combat Simulator) world is a game where you can fly in realistic combat aircraft. Normally the only aircraft available for free are the Su-25T Frogfoot and the TF-51D Mustang. The game includes training and regular missions, an online multiplayer. It has one free map and 3 additional maps.