This hack of Super Mario World. It doesn't change any of the levels, instead it changes the physics - the player is sent flying at top speed in whatever direction they face. Spin jumps are a useful way of staying in one spot, and with Yoshi there's a delay on turning around. Of course, in a TAS this would be much easier to control - the main issue would probably be the camera.
This hack was made by a guy named GlitchMr, as a contest for a forum he used to run. Whoever got through all 96 exits legitimately first would get a custom user name colour. He edited a value in the save file so that people couldn't just import a previous SRAM and say they did it. I spent the better part of an evening doing this, since I fell in love with it as soon as I saw the title screen demo. It's ridiculously hard to control in realtime, though. One notable trick is that when you're big, you can clip through one tile high walls. It's like when you duck into a one tile high space and stand up, except it applies to the lower half of Mario. I don't have any other pearls of wisdom about this, though.