Linus Akesson built an 8-bit synthesizer into an old electric organ:
All the original tone-generating parts have been disconnected, and the keys, pedals, knobs and switches rerouted to a microcontroller which transforms them into MIDI signals. Those are then parsed by a second microcontroller, which acts as a synthesizer.
Here’s Linus running through some of the features of his “Chipophone”:
His presentation video above lasts over seven minutes, but he gets right to the point at 0:24, demonstrating exactly why this DIY project is so great. (Also: Tetris at 2:22, Commando at 3:34, and MegaMan 2 at 5:30)