Half-Life: The Downfall of Evolution - Trailers
Two trailers are out for Half-Life: The Downfall of Evolution, a fan-film taking place in Valve's Half-Life universe.
Half-Life: Raise the Bar
Half-Life: Raise the Bar is a fan-made short film inspired by the Half-Life franchise.
Portal 2 - Boots
The science of the boots used in Portal 2 is explained in this latest video from Valve.