The Unexpected Philosophical Depths of the Clicker Game Universal Paperclips
I remember starting this game at mid-day and having no idea that I would end up building paperclips well into the night.What impressed me the most was how the "secret" gradually unfolded, so that you were already hooked and felt a compulsion to continue game play, even after it became clear that this wasn't the innocuous game you thought it was at first.It led me to build a clicker game that also has more to it than meets the eye.  (Shameless self-promotion starts here.) 
 Whereas Universal Paperclips explores the dangers of letting A.I. take over, my game (Unaware, https://unaware.pressbin.com) explores the moral status of people who exist in a simulation.