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 you save an image in the Sunras format, you will be prompted to choose a data formatting type. Run Length Encoded (RLE) is the default, and is recommended, because it compresses the file without losing any quality. Use the Standard format if you are uncertain whether another application will support it.