https://steamcommunity.com/games/546560/announcements/detail/3094470492881722385Half-Life: Alyx is still coming out in March but before then, you can play Half-Life, Half-Life 2, Half-Life 2: Episode One and Half-Life 2: Episode Two for free. Valve is also making Opposing Force, Blue Shift and Team Fortress Classic free to play.
Also, there is a giveaway for The Witcher Goodies Collection, which includes all OSTs, artbooks, comic books, The Witcher 3: Wild Hunt concert and more