I recently got an iPhone and have been having a good time playing with it. One of the cool applications I installed is called ColorSplash. Essentially it allows you to take or reuse an existing photo, converts it to black and white, and then allows you to selectively add color back in. Evidently this process is called selective desaturation.
It’s a little tricky to use at first, but if you zoom and move the image around, you can get pretty good results. Here are a couple of shots from my iPhone in which I used ColorSplash to bring out the colors of flowers.
First, wallpaper/background versions are available. Click an image to get the fullsize image suitable for download:
Here are the images as originally taken and modified: