Music is of course an activity extremely suited for visually impaired folks, as it relies primarily on hearing. However, one major stumbling block is the tuning of your instrument.
For people without absolute hearing, there currently does not exists a good solution. There is an iPhone app that I've heard mixed things about, and some desktop software that is not suitable for tuning in many situations away from the PC.
Most experienced players that I talked to learned to tune by ear, because normal tuners use some LEDs or an LCD display for tuning feedback. But even they indicated interest in an easy to use portable tuner.
This tuner takes input from a clip microphone or electric guitar, and drives a headphone with an easy to distinguish clicking sound that indicates how closely in tune you are.
The initial prototype used an FFT frequency-domain based approach common to many guitar tuners. The new version takes its core functionality as an opportunity to innovate on the algorithmic side of things. By modulating the input with a reference tone (much like your FM radio), the frequency difference between the input and reference can be extracted an synthesized into a clicking sound. This can be done in real time with very little CPU and memory.
The business plan for this project is very simple: Put it on Tindie and hope for the best. There are already people on the waiting list, so while I don't expect to get rich, I should sell at least a couple.