Valve has announced that all the major Half-Life games are currently free to play until Half-Life: Alyx releases. As such, PC gamers can go ahead and download these classic first-person shooters and play them for free.
Do note that this is a free to play and not a free to own offer. Therefore, you can only play the old Half-Life games for free until Half-Life: Alyx comes out.
The games that are currently available for free download are: Half-Life, Half-Life 2, Half Life: Episode One and Half-Life Episode Two. Unfortunately, Half-Life: Blue Shift and Half-Life Opposing Force are not included in this free to play offer.
Valve has also re-confirmed that Half-Life: Alyx will come out in March 2020. The game is set before the events of Half-Life 2 and the episodes, but the games share characters and story elements. Thus, the developers believe that the best way to enjoy the new game is to play through the old ones.
You can download the classic Half Life games from here.