Removing image white space

If you need to remove white space of an image (crop only the image w/o any background), try paint.net

http://www.getpaint.net/



(Image used for conveying the idea only from http://www.technologo.com/marketing/vsample-basic)

Some discussions on the technique.

http://forums.getpaint.net/index.php?/topic/7530-remove-the-white-space-of-a-picture-crop/

Is there any tool that allow this operation in one click?