Photoshop CS4 Content-Aware Scaling
What could work better than a demonstration? Above: an original shot that is 425 pixels wide. Below, the same shot, same width, but “Content-Aware” scaled to reduce the size of the unimportant bits, keeping the subject matter unscaled.
You can do the same thing without Photoshop. See my entry on Rsizr.
In Photoshop, the key to controlling what gets scaled and what does not lies in the creation pf an alpha channel to protect certain area. White is protected, black will scale. Use the button in the toolbar to select your alpha channel when you use Edit >Content-Aware Scaling.