http://www.seamcarving.com/
'Seam-carving' allows for images to be re-sampled while selectively and calculatively preserving areas of the images that are important to overall content. Instead of distoring or dropping random lines of information, the software searches for paths (that aren't necessarily straight) of pixels that it can drop whose removal will not effect the overall image.
Instead of re sampling the entire photograph, it might isntead opt for only downsizing the sky, where important data isnt present, and in doing so, maintain the size and emphasis on say, a person in the photograph, or a structure.
http://rsizr.com/ for a rou