There are many great Indie Games. My far favourite are the ones by 'yiotro' which seems to be a single (Russian?Ukrainian?) developer. Each is some kind of strategy/simulation mix and very cheap with simple graphics but a lot of depth. I discovered his work from seeing Antiyoy which either is so old that I got a demo of it in a ~2002 gaming magazine, or a copy of that old game. Just amazing fun all around and the only reason I don't have any of them on my phone right now is that I can't stop playing if I install them (similar to Unciv and Shattered Pixel Dungeon, both FOSS and excellent choices that were mentioned already).
Thanks a lot, learned something new today! Though the guy keeps pumping out great other games which seems original, so I maintain my suggestion.