Portal: Survive! is a live-action short film based on the Portal game, and its tie-in comic Portal 2: Lab Rat.
Lil Guardian Pyro
Lil Guardian Pyro is the winner for the best overall prize at this year's Valve Saxxy annual animated film competition.
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.