Photography web site Photojojo details how to get crisp, beautiful prints from your digital photos with Photoshop's Unsharp Mask. What's the point of sharpening, you ask?
Digital cameras have a fixed grid of pixels, each of which can only capture one colour or shade at a time. Say you take a picture that has a sharp edge between black and white... The single pixel that records that hairline edge can only record one colour, so it renders it as grey. What we think of as sharpness is actually the contrast we see between different colours. A quick transition from black to white looks sharp. A gradual transition from black to grey to white looks blurry.
A little time with the Unsharp Mask, though, and you can lose the blur along the edge of colour transitions and bring more sharpness and fidelity to your photos, and Photojojo's guide is an excellent starting point. If you're an expert Unsharp Mask-er, let's hear your tips for making the most of the tool in the comments.