Demos are great, and the Steam Game Festival: Summer Edition is probably the most extensive collection of free demos out there, with over 900 to choose from at the time of writing.
The third, and biggest-by-far Steam Game Festival runs from now until June 22 at 10am PDT / 1pm EDT / 6pm BST. Frankly, there isn't enough time in the event's duration to play even close to every demo, not to mention watch developer livestreams, interviews, and playthroughs.
Scrolling through the ever-expanding list, it's clear the focus here is on upcoming indie titles, but you'll also find demos for some older classics like Heavy Rain. As our friends at PC Gamer point out, the event page doesn't link to a dedicated list of every free demo, but you can use Steam's search function to reveal the list.
As mentioned earlier, Steam's Summer Game Fest isn't just a ton of demos; there are also several events happening every day during the event. Right now there are several dev streams and interviews scheduled each hour for the evening. Check the schedule here to see if there's anything you want to tune in for.
Correction: The article originally stated that "thousands" of free demos were available as part of the Steam Game Festival: Summer Edition. That number was based on the results from Steam's demo-exclusive search function, which we've learned yields some outdated pages without demos. The article has since been updated and we regret the error.
ICYMI: last year was a fantastic time for indie fans, so we wrote up a list of the best indie games of 2019 for you to play while you're stuck at home.