For anyone who hasn’t yet bought the Half-Life games on Steam, you can now play them all for free until the release of Half-Life: Alyx in March.
“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.”
The current free-to-play campaign includes Half-Life 1 and 2, as well as Opposing Force and Blue Shift. And Team Fortress Classic, if you’re into that.