A free program for Windows, by Red 5 Studios.

Firefall is a free-to-play, action-packed, multiplayer, virtual world simulator from Red 5 Studios. In the game, you’ll be able to build and command your own armies to conquer the world. The game is set in an apocalyptic world, where the energy storm called the Melding has recently occurred. It has caused a massive game of war, in which the players must band together to defeat the monsters called the Chosen. You’ll be able to use different types of weapons, armor, equipment, and vehicles as you wage war against the enemies.