VVVVVV is a retro styled 2D platformer by Terry Cavanagh, creator of dozens of free games. You play as the fearless leader of a team of dimension exploring scientists who inadvertently crash, and are separated. The game involves exploring the strange world you've found yourself in, and reuniting your friends. VVVVVV explores one simple game mechanic: you cannot jump - instead, you reverse your own gravity at the press of a button. The game focuses on playing with this mechanic in a variety of interesting ways. The game is designed not to artificially gate your progress. In VVVVVV there are no locks, no power-ups, no switches, nothing to stop you progressing except the challenges themselves.
Once I've thought about playing it, and it's 7 years after its release, I have played it in the Nintendo Switch version. It was very good, but it was very difficult until there were almost impossible parts or even completely, but this is solved with the invincible mode, and you spend everything. I would only mention two bad things, some serious bugs in passing phase by phase through walls of the edges and a giant elephant of epilepsy (but it's at the same time a curiosity).