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 - Authoring Tools Open Beta
The universally acclaimed Portal 2 is getting a level editor. Valve has launched an open beta for the game's authoring tools.
Portal 2: Lab Rat - Part Two
Read the second part of Portal 2: Lab Rat, the comic released by Valve as a tie-in into the new game in the series.