Witness what might have been had Nintendo (opens in new tab) had a few more bytes and hertz to play with in the '80s. Leveraging sickening talent and the dark arts, Trần Vũ Trúc has created an emulator (opens in new tab) that extrapolates 3D levels from the 2D sprites of NES games. And it runs in your browser (opens in new tab), provided that browser is Firefox.
There's a lot of speculation (opens in new tab) as to exactly how 3DNES does this. It's not as simple as extruding the layers of sprites backwards—some lines become loops in 3D, for example, and Dr. Mario's square pills look a bit like the heads of Lego men. It's no trick, though: you can plug in your own ROMs by providing a link to the emulator. Simpler games without detailed characters or fiddly text work best, as you'd expect.
While 3DNes only works in Firefox for the time being, the author promises an executable is on the way.