Oh wow, this thread is kind of dead.
Good thing I was looking for it, because I found a new thing :D
VVVVVV
https://thelettervsixtim.es/
It's certainly not the MOST obscure thing I've posted here, but I can't seem to find very many people who like it. In the FOUR DAYS after rediscovering the game, it has grown to be one of my top favorites. It's difficult--not as difficult as Catgrim, but still a tough game to beat. I should mention that the full game isn't free, it costs $5.00. You can play a demo on Kongregate, which you can get to using the link above.
Quoting directly from the demo...
"VVVVVV is a platformer where you cannot jump - instead, you can reverse your own gravity every time you make contact with a floor or ceiling. VVVVVV focuses on this one simple gameplay mechanic and explores it in detail. The game is designed not to artificially gate your progress. In VVVVVV, there are no locks, no powerups, no switches, nothing to stop you progressing except the challenges themselves. VVVVVV is by Terry Cavanagh, creator of the free games Don't Look Back, Pathways and many others. It features over half an hour of chiptune music by composer Magnus Pålsson. The full version contains lots of additional levels set in a huge overworld, as well as new game modes and a secret minigame."
EDIT: 9/21/2018
It has been twenty days since I bought VVVVVV, and I have quite a bit more to say about it now.
I love everything about this game. The plot is unique to other games I've played. It doesn't waste time with forgettable side characters; each of the six characters manage to have distinct and entertaining personalities. Even with the characters I'm not too fond of, it's more of a "I just like everyone else more" rather than "I specifically dislike you". The simple visuals and backgrounds reflect the gameplay and music perfectly; the simple controls are a huge part of why it's so good, as well. It's literally impossible to get confused by them. You move left, you move right, and you flip gravity. That's all, and it stays the same everywhere.
Even the room names are well thought-out and helpful, if you look into them enough. And some of the room names are just funny or clever, like Frown Upside Down and Doing Things The Hard Way.
And the game is very forgiving. Deaths don't send you back too far as long as you hit the checkpoints, and even in the Gravitron, you get a checkpoint every five seconds. So, yeah, it might take a while, but you will eventually complete it, as long as you keep Pushing Onwards.
The music is by far the best video game music I've heard. I can't begin to explain it. Pressure Cooker, Predestined Fate, and Presenting VVVVVV are too good for words. Positive Force is good, of course, but unlike a lot of other people, I don't think it's the best. That title goes to one of the three mentioned before, though I'm not sure which one. Even if you don't like the game, you probably will like the music. I've seen people buy the soundtrack without buying the game!
(And, even though it doesn't appear in-game, Waiting For VVVVVV is one of my favorites.)
I have so much more that I could say here, but I don't want to make this post too long. I'll leave you with this video of me "speedrunning" the game:
https://www.youtube.com/watch?v=LXoWtMwt0BU