It's been interesting to see how many new non-Photoshop image applications have been coming out for the Mac. While Photoshop can apply just about any effect to an image, I'm seeing an increasing number of specialized, low cost apps that do one or two things exceedingly well and don't require a mega-investment in Photoshop.
A case in point is Color Splash Studio. The app allows you to take a color image, turn it to grayscale, and then brush selected color back in. This isn't an effect you would use on every image, but it has its place and can look pretty amazing if the subject matter supports it.
Color Splash Studio is easy to use. You start