Update: And now it's official: the complete Half-Life Collection is free to play, at least for a while. In celebration of Half-Life: Alyx coming out in March, we're making the entire Half-Life Collection available to Steam users for free until launch day," Valve said in an announcement.

"While Half-Life: Alyx is set in a world that predates Half-Life 2 and the episodes, it shares the same characters and story elements, and the Half-Life: Alyx team believes the best way to enjoy the new game is to play the older games, especially Half-Life 2 and episodes. [The Half-Life Collection includes "Half-Life," "Half-Life 2," "Half-Life 2: Episode One," and "Half-Life 2: Episode Two," as well as the spin-off games "Opposing Force" and "Blue Shift", and the multiplayer "Team Fortress Classic".

Original:

Some Steam users received a notification yesterday advertising the Half-Life: Alyx event. The notice claimed that the Half-Life Collection would be free until the release of the VR spin-off, but there is no such thing as a Half-Life Collection, and all Half-Life games currently have a price.

While there is no bundle called Half-Life Collection, there are bundles of The Orange Box and Half-Life Complete, but neither is free. And despite the notice linking to an error page, Valve has made no announcement.

It appears that the notification was made too early, but that doesn't explain why the bundles have different names. It could simply be a mistake, and the notification may just refer to Half-Life Complete. It is also possible that Valve will eventually release a third bundle that includes Half-Life: Alyx.

I have contacted Valve to find out more about this event, so stay tuned for updates.