Valve have created Expiration Date, a 15 minute animated short featuring all the Team Fortress characters.
Half-Life Black Mesa
A relentless team of fans have finally released Black Mesa a remake/upgrade of the first Half-Life game from Valve.
Team Fortress 2 - ZombiU Crossover (Source Filmmaker)
This Team Fortress fan animation created with Source Filmmaker is a cool tribute to the ZombiU CG trailer.
Portal 2 - Boots
The science of the boots used in Portal 2 is explained in this latest video from Valve.