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).

I'm not a gamer, I play Tetris on my DS once or twice a year max, I just found this video (it's PC, BTW) and I've been watching the playthrough ALL day.  And it's kinda funky b/c I had been thinking not too long ago of drawing Mario in all these various other video games, and lo and behold not only did someone else do that but it's playable!

I used to play video games as a kid but I was never particularly good (I only ever beat Wario on Game Boy, though I think I came close to Mario Bros 2 on GB).  So these are really taking me back to my childhood.