Alien Swarm

Alien Swarm: Reactive Drop is a FREE and open source cooperative top down shooter. The player plays with other marines against hordes of aliens, in order to complete different objectives. In the game, there are 4 different classes to play, each one consisting of two characters with different statistics including some class restricted weapons. During gameplay, the player will earn experience, that is used to unlock new weapons and items to use in combat. The game combines elements from survival games like Shadow Grounds and Left for Dead.

Lambda Wars is a free standalone multiplayer Source Engine modification set in the Half-Life 2 universe that brings the war between mankind and their brutal Combine overlords to a traditional real-time strategy (RTS) setting.