Valve have announced the Source Filmmaker, their tool for creating animated films in a very intuitive way.
Half-Life: Raise the Bar
Half-Life: Raise the Bar is a fan-made short film inspired by the Half-Life franchise.
The first minutes of the original Half-Life are brought to life in this live action short, Half-Life: Origins.
Portal 2 - Boots
The science of the boots used in Portal 2 is explained in this latest video from Valve.