It may look like it was rendered on Ceefax but VVVVVV is one of those simple-to-learn-but-painfully-difficult-to-master platformers which I think of when someone says “indie game”. The “make and play edition” is a free version of the game without the official levels but with user-created levels and a level editor. Available for Linux, Mac, and Windows, though it’s FOSS so you can probably compile the mobile version yourself too!