A new chapter of the series is coming Half Life, although not the third as hoped by all the players in the original series. To celebrate then the arrival of Half Life: Alyx, Valve made the whole series free-to-play. The games are redeemable for free on Steam and will remain in your library until the next chapter is released, when they will return for a fee.
Then run to download immediately Half-Life, Half-Life 2, Half-Life 2: Episode One, Half-Life 2: Episode Two. Recall that the release of Half Life: Alyx, title exclusively in VR, is set for March. The exact day of its publication has not yet been announced.