NES emulator FCEUX might be the coolest thing to hit the emulation scene, mostly because of its support for Lua, an embeddable scripting language. It makes Super Mario Bros. cool and the sequel cooler.
It basically turns Super Mario Bros. 3 into a fairly capable looking Kirby's Canvas Curse clone, with a custom script that let's Mario "rainbow ride" on platforms drawn by a mouse. The script also disables the player's control of Mario, forcing the plumber to walk in one direction until he hits an obstacle.
Using the mouse, players must guide Mario to the exit, destroying enemies with lines of rainbow power and drawing ramps. A very clever modification to this classic game.