Portal: Survive! is a live-action short film based on the Portal game, and its tie-in comic Portal 2: Lab Rat.
Valve have created Expiration Date, a 15 minute animated short featuring all the Team Fortress characters.
Half-Life: The Downfall of Evolution - Trailers
Two trailers are out for Half-Life: The Downfall of Evolution, a fan-film taking place in Valve's Half-Life universe.
Valve's Source Filmmaker
Valve have announced the Source Filmmaker, their tool for creating animated films in a very intuitive way.