Few years back Valve released a free game called Alien Swarm - top down 3d multiplayer alien shooter for up to 4 players.
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Very polished and fun game.
Few days ago a group of fans(?) have released an extended version of the game, also for free. It adds steam workshop support, multiple campaigns, 8 player multiplayer, weapons, maps, aliens and more.
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Alien Swarm: Reactive Drop is available for free on steam.
I’d gladly shoot some green-blooded craps with you guys!