Synergy is a free cooperative modification for Half-Life 2 that transforms your favorite single-player campaigns into a shared adventure. By merging the core Half-Life 2 experience and dozens of user-created maps, Synergy lets you and your friends blast through City 17, navigate the canals, storm Nova Prospekt and more—all while staying together in the same world.

World of Goo is an award-winning puzzle and construction game built around physics, created solely by two people. In this game, you manipulate lively, animated globs of goo by dragging and dropping them to form various structures—from bridges and towers to makeshift cannons, dirigibles, and even enormous tongues. The Goo Balls, which populate the enchanting World of Goo, are driven by a natural curiosity to explore their surroundings, unaware that they exist only within a game—and that they happen to be surprisingly delectable.

Left 4 Dead is a cooperative first-person shooter that plunges you into a post-apocalyptic world where a vicious infection has turned most people into ravenous, violent creatures. In this game, you join a small group of survivors who must depend on each other to navigate through dangerous, abandoned environments overrun by relentless hordes of infected. Not only do you face the typical slow-moving adversaries, but you also encounter special, mutated foes with unique abilities that can suddenly turn the tide of battle.