Those with an artistic bent have been making color-isolated photos in Photoshop for years, but a recent crop of quick and easy tools for doing just that has allowed novices (like me) to create some of the same beautiful photos in a fraction of the time.
ColorWash, currently US$0.99 in the Mac App Store, is one such program. It allows you to make color-isolated photos in minutes. You load your image of choice into the app, which will then remove all color from it, giving you a grayscale photo canvas. You then 'paint' the color back in using a brush tool, restoring the original colors into the areas of the photo that you want to.
If painting the colors back in sounds like hard work, there's also an automatic fill tool to detect edges and colors and either fill in or remove the color automatically from a selection (although we had mixed results using this tool).
ColorWash is great for anyone who just wants to play around with their photos. But those looking for more advanced features, options and control over the image, or even just pinch-to-zoom, should probably look elsewhere.