Partha bundles some popular plugins with Gimp, so you don't have to hunt for them or worry about their compatibility.
Most of the open source RAW processors (Rawtherapee, Darktable, Photoflow...) are discussed on
pxls.us. Since they are free you can try them all
![Smile :)](./images/smilies/icon_smile.gif)
This is really the best forum if you are interested in photography and processing with open source software.