Half-Life: Alyx Released

Half-Life: Alyx is now available on Steam, offering a new virtual reality game set between the events of Half-Life and Half-Life 2. This requires a VR headset, and is free for owners of Valve's Index. There's also an article on Steam Support on streaming the game, since a headset is not required to spectate. Here's word on Alyx:
Half-Life: Alyx is Valve’s VR return to the Half-Life series. It’s the story of an impossible fight against a vicious alien race known as the Combine, set between the events of Half-Life and Half-Life 2.

Playing as Alyx Vance, you are humanity’s only chance for survival. The Combine’s control of the planet since the Black Mesa incident has only strengthened as they corral the remaining population in cities. Among them are some of Earth’s greatest scientists: you and your father, Dr. Eli Vance.

As founders of a fledgling resistance, you’ve continued your clandestine scientific activity—performing critical research, and building invaluable tools for the few humans brave enough to defy the Combine.

Every day, you learn more about your enemy, and every day you work toward finding a weakness.

ABOUT GAMEPLAY IN VR:

Valve’s return to the Half-Life universe that started it all was built from the ground up for virtual reality. VR was built to enable the gameplay that sits at the heart of Half-Life.

Immerse yourself in deep environmental interactions, puzzle solving, world exploration, and visceral combat.

Lean to aim around a broken wall and under a Barnacle to make an impossible shot. Rummage through shelves to find a healing syringe and some shotgun shells. Manipulate tools to hack alien interfaces. Toss a bottle through a window to distract an enemy. Rip a Headcrab off your face and throw it out the window.