Half-Life: Raise the Bar is a fan-made short film inspired by the Half-Life franchise.
Valve have created Expiration Date, a 15 minute animated short featuring all the Team Fortress characters.
Valve's Source Filmmaker
Valve have announced the Source Filmmaker, their tool for creating animated films in a very intuitive way.
Portal 2: Lab Rat - Part One
Read part one of Portal 2: Lab Rat, a comic created by Valve to bridge the old and new game in the series.