Half-Life: Raise the Bar is a fan-made short film inspired by the Half-Life franchise.
Street Fighter: Assassin's Fist
The live-action Street Fighter: Assassin's Fist series has been released in its entirety on the Machinima Youtube channel.
Valve's Source Filmmaker
Valve have announced the Source Filmmaker, their tool for creating animated films in a very intuitive way.
Portal 2 - Boots
The science of the boots used in Portal 2 is explained in this latest video from Valve.