How did I not know about this until today? This is a game built on the SMB3 Hello Engine 3, it starts in the Mushroom Kingdom with levels inspired by all the games (even the original Game Boy game), then goes into Sonic The Hedgehog, Megaman, Castlevania, a whole chunk of nightmare rounds, and there's levels like Prince Of Persia (!), an Arkanoids level... I wasn't sure which video to post, but here's Mario going into Halo World (there's a Metroid world as well).
http://www.youtube.com/watch?&v=IigRy1-PIKk
Here's some very cool levels from the Nintendo Subspace world. Love that there's a level dedicated to the Game & Watches.
http://youtu.be/mNlPSUMVJJ8