To promote the upcoming game, you can now play the Half-Life, Half-Life 2, Half-Life 2: Episode One, and Half-Life 2: Episode Two games for free on Steam until March 2020 when Half-Life: Alyx is released.
"Half-Life: Alyx is set before the events of Half-Life 2 and the episodes, but the games share characters and story elements", Valve said in a news post.
This week, Valve decided to make the entirety of the Half-Life series of games available to play for free through Steam. Valve has partnered with Steam to offer all the previous Half-Life titles free for the next two months to Steam account holders. Half-Life: Alyx is now set for a March release window, although no exact date has been specified. For example, just before Christmas, Half-Life and Half-Life 2 were in the sale at £0.71 each, though they are now listed at £7.19 each...
The Half-Life games regularly go on sale, but if you've never played them before, now is the flawless time. The issue? Well, for one thing, there isn't any title out there called the Half-Life Collection. The official Half-Life games are only free for the next two months, which is around the time Half-Life: Alyx will release.
The Half-Life games might seem less remarkable to newcomers after decades of other studios following Valve's example, but even if you don't click with the first I would still recommend trying HL2.
Do note that this is a free to play and not a free to own offer. Now you can get up to date on this older series before it makes its big comeback. At the very least, it's probably worth playing through Half-Life 2 again so that Alyx, which is set before Gordon Freeman arrived in City 17, will make sense to you.