From Half-Life 2 to Portal 2: The Visual Evolution of Valve's Source Engine
In 2004, Valve released their proprietary Source Engine for use in modern games. Valve showed off the capabilities of the Source Engine impressively when they released Half-Life 2. Since then, all of Valve's games have been powered by the Source Engine - and some titles from other developers as well. See the Visual Evolution of Valve's Source Engine from Half-Life 2 to Portal 2 in the following gallery.
Note: Click inside the picture for the high resolution version.