Valve has announced that every game in its seminal Half-Life series is now free to play on Steam - but only until the release of Half-Life: Alyx later this month. The exact date for the new VR game hasn't yet been announced, so you could have anything from a few days to a few weeks (much more likely imo) to take advantage. The company blogpost says:
Half-Life: Alyx is set before the events of Half-Life 2 and the episodes, but the games share characters and story elements. The Half-Life: Alyx team believes that the best way to enjoy the new game is to play through the old ones, especially Half-Life 2 and the episodes, so we want to make that as easy as possible.
From now until Half-Life: Alyx launches, visit the game page of each Half-Life game to install it and play as much as you want, right up until the day Half-Life: Alyx is released!
The games include Half-Life 1 & 2, both Half-Life 2 episodes, as well as the story offshoots Half-Life Opposing Force and Half-Life: Blue Shift, with the multiplayer-focused Team Fortress Classic completing the package. The wording makes it clear that this is not a complete giveaway: if you're halfway through one by the time Alyx launches, it looks like hard cheese.
Half-Life 2 and its expansions remain one of the greatest FPS experiences around and, while the original has been a victim of time's ravages, it was a standout title of its era. So rise and shine, Mr Freeman. Rise and shine.