Drawing Rectangles in GIMP

Select a rectangular region by clicking "Rectangle Select Tool". Then click on Edit -> stroke selection. Inside this tool, you can select different line styles like continuous, dashed etc. In order to change color of the line, click on "Foreground and background color" item available in Gimp's main menu. More elaborate discussion is available in the following link:

http://pbs01.wordpress.com/2007/09/30/145/