The Half-Life games regularly go on sale, but if you've never played them before, now is the flawless time.
The games in the limited-time Steam promotion include Half-Life: Source (a modern-ish remake of the original game), Half-Life: Opposing Force and Half-Life: Blue Shift (the two expansions for Half-Life), Half-Life 2, Half-Life 2: Episode 1, and Half-Life 2: Episode 2.
The preceding Half-Life titles were immensely popular on release, and Valve often discounts them during its Steam sales, too. The first game debuted in 1998, then Half-Life 2 was released in 2004, followed by Half-Life 2: Episode One and Half-Life 2: Episode Two in 2006 and 2007, respectively. However, because these games share some story elements and characters, Valve wants people to be able to play the previous games first. Make sure you click on the free link rather than end up inadvertently purchasing any of the titles. But Valve making all Half-Life games free could still help drum up enthusiasm for Half-Life: Alyx.
Fans may very well want to connect the plot dots between the biggest Half-Life adventures ahead of HL:A's launch. Thus, the old games are being made available for everyone to play for free before the release of the new game so that players can get maximum enjoyment out of the new game.
Valve has also re-confirmed that Half-Life: Alyx will come out in March 2020. Set between the events of Half-Life and Half-Life 2, in the game Alyx and her father Eli "mount an early resistance to the Combine's brutal occupation of Earth". You can pre-order it now for £41.84 instead of the standard price of £46.49.