Trochoid is the App, which is free. They have a pro version, and there appears to be a handful of other iphone Apps that can generate this type of Spirograph-like art. The very fluid response is fun to experience in real time as you're moving the ratios around with your fingers, There are color variations with this, although I found myself gravitating towards a monochromatic scheme, so I could see the patterns more clearly. Usefulness? Not so sure, but you can save them to the photo album, which is becoming the main qualifier for me. Here's a sampling, albeit a monochromatic one. The straight lines and triangles were a bit of a surprise.