Excitebike riders, Kirby, and Samus are trying to stop you at every turn from reaching the end of the levels! Here is a full game hack which changes all the levels and graphics. Also the music has been slightly altered to give the original music some change.
Not a ROM hack, but Redigit, before going on to make Terraria, made something called SMB X, remixing SMB 1, 2, 3, World, and Zelda 1 and 2. It also had its own level editor. Totally fun.
I don't remember the name of it, but it was just a super troll hack where you're presented with the normal game for Mario 1, but then every obstacle had a hidden obstacle that would kill you when you attempted to play it as if it was the normal game.
There was another game made around the same time that was built entirely on that premise, but I'm pretty sure the Mario hack came first. I at least played it before I ever heard about I Wanna Be The Guy.