[Frogatto & Friends is free and open source on Windows and Linux, on the other platforms it costs $4.99].
Frogatto & Friends is an old-school 2D platformer game, starring a certain quixotic frog. Give it a try! It's a big adventure with all the classic fun - you fight monsters, collect coins, talk to people, and buy new stuff. Also, frogatto has a very flexible game engine you can use to make your own creations.