Developer Jay Pavlina just released version 2, and it's a heartbreakingly beautiful work of retro-gaming. Now there are multiple versions of all the characters, including NES, SNES, and Game Boy styles, and you can cycle randomly through versions of all the levels in those styles.
Best of all? Among those styles are Demon Returns and Super Demon Returns, the Super Mario Bros.-style game from the DS's Retro Game Challenge 2. So you can totally play as the tank from Blaster Master in Super Mario Bros. levels that look like Retro Game Challenge.
The characters include Mario, Luigi, Link, Samus, Simon Belmont, Mega Man, Bass, Bill Rizer, Ryu, and Sophia the 3rd -- all with alternate looks, and character-appropriate special abilities. Did we mention this runs in your browser, and that therefore you could be playing it right now?