Heavily scripted shooters that take place in glorified corridors are well and good for most, but some people like to kill things out in the open, and kill them with a single bullet. For them, there is ArmA series, and for them, a third game in the franchise is on its way.
ArmA III expands on the same basic premise of the last two games, in that it takes place on a large island and gives you access to every inch of said island, along with the ability to do anything from run around shooting a pistol to climbing in a chopper to driving a tank.
In addition to a singleplayer campaign, there's the expected multiplayer modes (with both "cooperative & competitive scenarios") and mod tools. You can also create dedicated servers on both PC and Linux.
It'll be out next year.