The Sharpen Tool in Photoshop has always been useful in that it allows you to quickly sharpen specific areas in a photograph, but a major problem was that it had the tendency to introduce nasty artifacts into the image. Alternative methods that avoid this issue (e.g. using a new sharpened layer) became popular, leaving the Sharpen Tool to gather dust on many users’ tool pallets.
Well, if you’ve recently upgraded to Photoshop CS5, you might want to take another look at the tool. They’ve quietly introduced a new feature (on by default) called “Protect Detail”, which allows for brush-based pressure-sensitive sharpening without the annoying artifact problem.