Is there an easy way to create a "saturation-map" in Photoshop/Gimp? I need a grey-scale version of an image showing where the colors have less saturation (black) and where they have more saturation (white).
You want to convert your image to a HSV/HSB model (Hue, Saturation, Brightness). The saturation is what you're looking for.
Photoshop cannot do this natively, but you might find a plugin that does that for you. The GIMP has this built in, just open the image and select