R/GameDeals/ often has posts every time there's something notable that's free for a limited time. You can still get them if you do it today, the promo expires on the 30th. However, not everything is available on Steam. In April they gave away Steamworld Dig 2 and Tales from the Borderlands. If you have Amazon Prime/Twitch Prime, they give away games at times as well. GOG offers them every now and then (I remember they gave away Oxenfree in December). If you grab it for free right now, at the steam store, you get to keep it forever. The caveat is that they're usually a bit older, banking on people already owning them, and you need to get them within their promotional period (they give you a Steam key and you keep the game forever. They give away multiple free games per month, many of which are single player and great. In terms of plot, it’s surely one of the most intriguing free single-player games on PC we will mention here. Sign up for the Humble Bundle newsletter. You must discover why Robert (the main protagonist) was targeted by an unknown enemy and almost killed. If the purpose of the question is to find great free single player games to play, and not to literally inquire what the best always-free single player games are, then I can help with a few ideas.

Good free single player games on steam