These preview environments are available for free to those who already have or have ordered Valve Index headsets
(which come bundled with a free version of Half-Life: Alyx).
One of these environments is an outdoor area in City 17 at the base of the in-construction Citadel; the other is
the laboratory of Russel, a main character in the game.
Valve does note, however, that
the rendering technology available in SteamVR's native environments is different than that in Half-Life: Alyx, and the interactivity is significantly lower than what the game itself provides.
Half-Life: Alyx is Valve's first Half-Life game since 2007, marking a long-awaited return to the series. According to Valve, the game's plot follows a younger version of Alyx (with apparently no Gordon in sight) and takes place between the events of the original Half-Life and Half-Life 2 – so it's no Half-Life 3. However, Valve recommends that players should have already played through the two episodic sequels to Half-Life 2 before playing through Half-Life: Alyx.
Half-Life: Alyx is available for pre-purchase on Steam before its 23 March release date, on sale for US$54 (NZ$69). In preparation for its release, Valve made the entire Half-Life franchise available for free on Steam, so now's a good time to freshen up on the plot of Half-Life 2 and its episodes before the release of Half-Life: Alyx.
Watch the announcement trailer for Half-Life: Alyx below!