Concert Keyboardist is an application I developed for my own use, in part because I have less than perfect eyesight in both eyes.
Being a music "sight reader" this limited my ability to play. Being a software developer I wrote some software to provide a crystal clear display that I can see and play from.
It turned out to be so much fun to play that decided to make it available to the public for free. It was written in JUCE so I was able to make it available on both windows and Macintosh.