rsizr is a very innovative web-based application to intelligently resize images, without loosing much details. Instead of dupkicating or removing some columns or rows of pixels from an image, it uses a technique called Seam Carving to duplicate seams in an image that contain most details, or to remove the seams that contain least details. Even Photoshop (c) does not have that capability, yet.
Seam Carving Technique:
rsizr in action:
The user interface is very simple. Simply put, you upload your image (make sure the size is suitable for your speed, it may help to resize your image before you upload it). After that, you can mark the areas you need to protect (green), and the areas you want to remove (red) and then start your resizing. It is that easy. Give it a try, it is a lot of fun removing that person from the group photo!