While Scanner Sombre is technically an exploration driven game with story and puzzle elements, not completely unlike several of the other entries on this list, it truly tests your ability to rationalize your location in a 3D environment. This is thanks to the main mechanic: you play in a completely pitch black environment with only your lidar gun to light the way with a spectrum of rainbow colored dots. (You should really watch the trailer if you want to get how this works.)
This can, especially at first, be somewhat disorienting, but as your mind adjusts it allows you to better concentrate on your journey. Along the way, a tale of miners and ancient rituals is also unveiled, but shhh! I've already said too much.
Scanner Sombre is available on Steam for $9.99. It is also compatible with Oculus Rift and HTC Vive.
And there we have it, with Scanner Sombre I present the last of the 6 unique indie titles. Some test your brains, some test your reflex skills. Some educate, some nauseate. There's something here for every type of gamer, of course, that is assuming you don't mind haunting experiences with graphic imagery.