U-bow

With this app, you can select from a number of string instruments and bow the strings just by touching your fingers to them. The sounds are entirely realistic; the engine is designed to mimic the actual physics of a vibrating string as closely as possible. You can play clean or distorted, in combination with a fair number of tone presets to choose from. Whether or not you find an actual use for this app, I guarantee you that you will have fun playing with it.