I love juggling, and have been trying to master 5 balls for quite a while. I really wanted an app that could track my progress, so that I could see if I was improving over time. I decided to take advantage of the accelerometer in the Pebble watch, so that I could track the number of throws, and times between throws.
We built a Pebble / iPad / iPhone application that records your juggling sessions, and shows you metrics such as total throws, and the average height of each throw. It also graphs your progress over the session, so that when you start to slow down, you know it's time to take a break. I'm most proud of the calorie tracking feature, since we know all the details about every throw. Juggling is not typically considered an intense workout, but you can definitely burn a non-negligible number of calories during an hour-long session. My goal was to accurately calculate that number, and push it to other services, such as MyFitnessPal (as soon as they open their API - we didn't have time to reverse engineer it this weekend.)
Our target user-base is myself, of course, and the niche juggling market. There are no apps out there that currently do anything like this, so with a little marketing, I believe that there are thousands of jugglers who would love to use our app. And of course, it could be a great way to learn to juggle if you're not already into it.