Since I moved from Windows to the Mac a couple months ago I've been trying out various Mac applications. I'm particularly enjoying the range of photography apps I now have access to.
One app I like is iSplash (US$0.99 in the Mac App store) which makes a photo black & white except where you choose for the original colours to remain. It only takes a minute or two to process a photo.
It creates simple yet astonishing effects like this. Yes, the flower was that yellow. But how much more yellow it looks with the surrounding colour removed.