A good example is ColorSplash [App Store link] a $1.99US app that brings some of the sophisticated controls you'd find in Photoshop right onto the iPhone. ColorSplash can take a color image from your camera roll, or any image uploaded to your iPhone/iPod touch, and desaturate the photo selectively. You can have the photo all grayscale, except for a face, or take they sky to grayscale while leaving the rest of the landscape in color.
You can zoom into the photo using the usual finger controls, and paint color selectively on an image, leaving a very striking effect. You do all this with your finger on the touch screen, and with a bit of practice and judicious zooming you can really be very accurate. We're used to seeing images like this come out of high end photo programs, but doing it on the iPhone is a bit of a stunner.
The app allows unlimited undo, and brushes can be semi-transparent, so the amount of color can be varied. A palette gives you the option of hard edge or soft edge brushes, and you can save your work at any stage and go back to intermediate versions of your image if you want to. You can work in portrait or landscape mode, and there is detailed, built in help.
The program author, Hendrik Kueck. is a computer scientist from Vancouver, B.C. who contributed to the creation of Adobe Lightroom 2.0, so he's no slouch when it comes to imaging theory and practice. He previously did Juxtaposer for the iPhone and iPod touch.
The best way to see what the program does is look at some of the simple images I did in the gallery below, or click over to the author's web site for some more information.