The GIMP can both read and write images in the Sunras (Sun Raster) format. The format supports grayscale, indexed, and truecolor images. As the name suggests, the Sunras format in mostly used by Sun applications.
When saving an image in the Sunras format, a prompt to choose a data formatting type will be presented. Run Length Encoded (RLE) is the default. It is recommended because it compresses the file without losing any quality. Use the Standard format if there is any uncertainty as to whether or not another application will support it.