Hyperspeed Slide Kicking
Mario is usually placed on the ground in the concave corner formed by 2 walls, so that he is running into the unreferenced one. By repeatedly slidekicking and recovering, Mario can build up a maximum of ~95 speed on normal floors and ~400 speed on slippery ones (at 400 speed he will break through the wall).
Mario can also be placed on a shell clone instead of on the ground, as shell clones allow him to slidekick instantly without recovering. Since he does not lose speed from recovering, the maximum speed achievable is again limited by the wall, so he can build up ~400 speed.