GIMP Plugin Registry

This technique / effect is called vignetting. Its a great way to learn the basics of the quickmask. (If you work with layers, -- or wish to learn how to -- you can expand the technique by using transparency and a layer above or below the main layer so you can fine tune the image more than you can with just quickmask.)

There are a number of good tutorials on how to achieve the basic effect. One can be found at:
http://www.gimp.org/tutorials/Quickmask/
Good luck!