I usually avoid posting free stuff on Humble or any other site that requires much more than a click to activate because those usually take more than a click to activate. I also can't find any free weekend announcements in the forum search so let this be a first. But I think this is a worthy candidate to be so.
What is an Alternate Reality Game (ARG)? Basically it's someone acting as if their narrative was real. It can be used as a promo tool or just a fun story stomeone wants to tell. It feels like some ARG have a spooky element involved but most involve puzzles. It's mostly stuff like the notpron levels.
Now the only example of this kind of game on Steam I know is The Black Watchmen.
This is NOT a permanent free game. This is a free weekend with discount attached.
What this also is not would be a game where you lean back and just "play". Expect tabbing between the game, a browser, image editing software and whatever else you might need. And should you get immersed, maybe also prepare for live action events in this real world thing your friends keep telling you about when they see you hang out with your better friends on TU.
See you on the field, agent.