These are the games listed FOR FREE: Abzu, Enter the Gungeon, Rez Infinite, Subnautica, The Witness, Astro Bot Rescue Mission, Moss, Thumper, and Paper Beast
Also do note that you will keep the game forever if you claim it, unlike those Steam "FREE WEEKEND" Sales. Keep in mind that Horizon Zero Dawn Complete Edition is also free to download from April 19 to May 14.