The Optigan was a 1970's organ based on optical rotating discs. In fact, it was much more than just an organ: it was more like a sampler. You could even play pre-recorded beats with it and more. The problem is that the Optigan is very rare nowadays. However, the discs are still available...
...so Gan at gaje.jp decided to make his own Optigan. Very impressive!
Here's the video!
[Found via Hackaday]