http://www.digitalcombatsimulator.com/en/world/

DCS World
[DCS World] Digital Combat Simulator World (DCS World) is a free-to-play digital battlefield game, focusing on a military aircraft simulation. It includes a FREE Su-25T and TF-51D aircrafts and it is extensible through additional DCS modules as well as user-made add-ons and mods.

DCS World includes a vast mission area of the Caucasus region that encompasses much of Georgia - the location of the Russian invasion of Georgia in 2008. Additionally, DCS World includes:

    Powerful mission and campaign editors
    Multiplayer
    Massive inventory of air, land and sea combat units and weapons
    Advanced AI
    Fast mission generator
    Dynamic weather and seasons
    Training missions
    Mission replay system
    Pilot log book

This all allows the creation of engaging, real-world combat missions in this flashpoint region. DCS is a true "sandbox" simulation that can and will cover multiple time periods covering many types of combat and civilian units. DCS World allows both realistic game play and more relaxed game play to suit the player.

DCS modules that can plug into DCS World can include aircraft, maps, ground units, campaigns, etc. Not only can DCS World include modules developed internally by Eagle Dynamics, but it can also include those by certified third party developers.  

DCS World includes a FREE Su-25T "Frogfoot" and non-armed TF-51D Mustang aircrafts!

Other modules that can be purchased for DCS World include:

DCS: A-10C Warthog. Warthog brings the most realistic PC simulation of a modern fixed wing combat aircraft in regards to flight dynamics, avionics, sensors, and weapon systems. Warthog is considered by many to be the current bench mark of PC flight simulation.

DCS: Ka-50 Black Shark 2. The Ka-50 "Black Shark" is a unique and deadly single-seat, Russian attack helicopter that has seen combat in the Northern Caucasus. It combines a high performance dual rotor system with a deadly weapons payload of guided missiles, rockets, bombs, and a 30mm cannon.

DCS: P-51D Mustang. The most realistic simulation ever done of this legendary World War 2 ear fighter.

DCS: Combined Arms. DCS: Combined Arms gives you control of ground forces during the battle. Use the Command Map to move ground forces, set artillery fire missions, and control the ground battle. Assume the role of a Joint Terminal Attack Controller (JTAC) in multiplayer and designate targets for player-controlled close air support aircraft, or directly control armor vehicles or air defense weapons and engage the enemy. 

DCS UH-1H Huey and Mi-8MTV2 helicopters by Belsimtek.