Liquid Rescaling or seam-carving, as discussed in this paper (pdf), is a method developed by Shai Avidan and Ariel Shamir that:
can change the size of an image by gracefully carving-out or inserting pixels in different parts of the image…
A seam is a connected path of low energy pixels crossing the image from top to bottom, or from left to right. By successively removing or inserting seams we can reduce, as well as enlarge, the size of an image in both directions. For image reduction, seam selection ensures that while preserving the image structure, we remove more of the low energy pixels and fewer of the high energy ones. For image enlarging, the order of seam insertion ensures a balance between the original image content and the artificially inserted pixels. These operators produce, in effect, a content-aware resizing of images.”
What this means is that if you resize a photo, using liquid rescaling, its rescaled while retaining all the important parts of the photo but without distorting it. If that doesn’t quite make sense then check out this video.
And now there’s a plug-in for the GIMP image editor you can download and install to get liquid rescaling on your desktop. Currently the plug-in is not as speedy, nor as functional, as seen in the video but then its only at version 0.2.1 so it should improve.