Half-Life: Source is a port of Valve's original acclaimed game Half-Life to their new Source engine used for Half-Life 2 . It was initially only available in the Silver and Gold Edition Steam packages when buying Half-Life 2, and in the Collector's Edition retail disc package. Later, it could be bought separately through Valve's Steam game delivery system.
It takes advantage of vertex and pixel shaders to create realistic water effects, and can now portray realistic physics effects. It also has new menus, improved lighting and soft shadows, ragdoll character animations, and a 3D skybox replacing the old 16-bit color prerendered bitmap skies.