Seam Carving is perhaps the the most interesting image resizing technology out there. It works by changing the height and width ratio of the image without distorting the image itself by croping the image. You can learn more about Seam Carving by Ariel Shamir on his website:
Now Seam Carving it possible with Free, Opensource, Multiplatform Image editing softwar, Gimp.
The plugin (called Liquid Rescale) can be downloaded from here source/Linux/Windows:
Seam Carving (Liquid Rescale) in action:
Might not be the best example, but without tweaking much with the option and by just changing the width of the picture here is a before and after example of the plugin: