Natural Selection 2

Natural Selection 2 runs a first-person shooter and a real-time strategy game on the same map at the same time. Human Frontiersmen face the alien Kharaa, and each side answers to a Commander who watches from above, placing structures, gathering resources, researching tech and triggering abilities while everyone else fights at ground level. A marine Commander might drop ammo and sentry guns onto a pinned squad; an alien Commander might grow a new Hive to spread infestation and open more spawn points across captured ground.

The two factions play nothing alike. Alien players evolve into one of five lifeforms: the wall-running Skulk and its lethal bite, the gas-spreading flying Lerk, the Gorge that heals allies and builds tunnels and turrets, the Fade that blinks through fights with scythes, and the enormous Onos that whole squads struggle to bring down. Marines answer with rifles, shotguns, grenade launchers and flamethrowers, and can build minigun-toting Exosuits or strap on jetpacks to storm a Hive.

Developer Unknown Worlds shipped the game with the full toolset used to make it, and all of its code is open source. Mods can be published, shared and pulled from the Steam Workshop, and they download automatically when you join a modded server.