This crazy game was made by Avalanche Studios and currently there is a big discount on it on Steam!
Now to make things a bit more clear, it's like a combination of GTA, tomb raider and farcry.. There is a lot of flying, shooting, parachutes, airplanes, choppers, huge guns, you name it.. There is also talk of a working multiplayer mod for this, how good is that?
Get the game here while it's still on discount! http://store.steampowered.com/agecheck/app/225540/?snr=1_4_4__139