Some of the coolest Super Mario Bros. levels can only be played with a method that many probably never even knew existed.
Nintendo's e-Reader, released in 2002, wasn't the type of gadget the name refers to today. It hooked up to the Game Boy Advance and consumers could buy trading cards to swipe through it to play full NES games or to add content to existing games. The e-Reader didn't really take off, with many dismissing its capabilities, but those people probably never saw these e-Reader exclusive Mario levels.
Nintendo sold packs of cards that when swiped unlocked levels in Super Mario Advance 4: Super Mario Bros. 3 that contain elements from all of the Mario games mashed together, or even totally new ones. Remember the cape powerup from Super Mario World for the SNES? The e-Reader brings it to Super Mario Bros. 3, along with items to dig up from Super Mario Bros. 2, the steel lifts from Super Mario Bros., and countless other level design elements and enemies from most of the Mario titles.
These levels can only be played by using e-Reader cards that are sure to be nearly impossible to find now. 12 cards were released in the U.S., with another 24 in Japan, for a total of 36 unique levels that the majority of people have never touched. From the video here showing the mashed-up elements, they look like something Mario fans might want to hunt for. I know I'm going to unearth my e-Reader and see if I've got any of the cards lying around.