Gimp script: Selection area finder
I just wrote a little GIMP script inspired by the discussion here: http://www.cartographersguild.com/general-discussion/2279-looking-free-area-calc-software.html
It will tell you the perimeter of the current selection in pixels and the area as a percentage of the total image area.
Installation and use: Once you've installed the .scm file in your gimp scripts directory, "Area and perimeter" is accessible under "Select" from the image menu.
UPDATE: Version 0.2.0 - added perimeter-finding