Half-Life 2

Half-Life 2 was not quite the debut of Source, but it was the first time the engine's full power was demonstrated. The player again picks up the crowbar of research scientist Gordon Freeman, who finds himself on an alien-infested Earth being picked to the bone, its resources depleted, its populace dwindling. Freeman is thrust into the unenviable role of rescuing the world from the wrong he unleashed back at Black Mesa, and a lot of people he cares about are counting on him. Half-Life 2 was revolutionary in 2004 for its physics, AI, and graphics.