One of my favorite Daedalic point-and-click adventures is free on Steam, and a pile of others have been marked way down
Daedalic was widely viewed as the inheritor of LucasArts' point-and-click adventure crown a decade or so ago, back when it was putting out some truly great point-and-click adventure games. Whether Deponia is one of them is a matter of some debate, but I think it is, and more importantly I know that you can pick it up for free right now on Steam.
The Deponia trilogy tailed off pretty sharply through the sequels and I honestly wouldn't recommend them, but I really enjoyed the first: A slapstick...