Built my own app for tracking my exercises using a phone arm band, the gyro, accelerometer, and the camera to create a model for rep counting and tempo. I got tired of the mental effort of counting reps. This graphs the strongest signal using an fft and some custom algo to determine the best signal to use. Works pretty well and backs up my own internal count. I promised myself I would only build it for me so I would actually build it as opposed to getting a mental block by building it for others.
My long term vision was to open it up to others and mine the data to determine what the ideal weight, tempo, resting period, and exercises were for a particular body type. I'm just a little too ADHD to commit to it and keep a day job.