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.
It's been well over a decade since the release of Half-Life 2: Episode 2, and with Half-Life: Alyx - the enormously long-awaited next instalment in Valve's seminal FPS series - mere months away, there are likely more than a few people in need of a story refresher.
Half-Life: Alyx is Valve's virtual reality title and is slated for release in March. 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... If you play them and decide that you want to own one or all of Half-Life series, then you'll have to wait until they're no longer free to play to purchase them.
Half-Life: Alyx is scheduled to be released in March 2020 and is Valve's first title geared entirely to VR play on the PC.
There isn't anything on Steam, at the moment, called the Half-Life Collection. The rest, as they say, is gaming history and Half-Lif quickly grew a cult following that generated fan-made content, both short videos and mods, for years to come.
As Alyx, you will engage in combat, solve puzzles, and explore a world turned derelict thanks to an alien invasion.
It even has Team Fortress Classic included in the set just for kicks, even if it's not directly related to Half-Life. With the right mods, all these games really hold up and are well-worth experiencing if you didn't catch them the first time around in the early Aughts.