ColorStrokes is a cool little US$0.99 app for the iPhone that lets you selectively add or subtract color from an image. Like most image editors and filters, ColorStorkes can tempt you to go too far and overuse the effect. But in the right hands, an image can become striking and artistic. It's not the software alone, it's the user that makes the image a memorable one.
ColorStrokes gives you a very nice set of tools for manipulating an image. Because the editing is done with your finger, and because your finger would cover the area where you are trying to do delicate work, the app helpfully provides a new window that lets you see what your brush is doing. It works really well, and along with a zoom and pan tool, you can really work in very small areas.