Valve have created Expiration Date, a 15 minute animated short featuring all the Team Fortress characters.
Counter-Strike: Global Offensive - Trailer
Valve show a grittier more realistic use for Source Filmmaker with a trailer for Counter-Strike: Global Offensive.
Portal 2 - Boots
The science of the boots used in Portal 2 is explained in this latest video from Valve.
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.