Fluid Piano – an acoustic piano with an microtonal interface
http://www.thefluidpiano.com/the-fluid-piano/index
This has been a labor of love for the inventor/developer Geoff Smith. It is pure genius (imo), and while very narrowly focused in it’s use, it exclusively fills a needed position in microtonal performance pianos. There are other videos showing it in performance.
ADDED : Seven and a half years later, watch this video ( from around the time of this original post ) to understand the joy of the Fluid Piano.