An early influential Google Maps mashup now gives users the chance to change mapping imagery. The GMaps Pedometer (our GMaps Pedometer mashup profile) has six different tile options, the most recent of which is OpenStreetMap, the free editable map of the world.
It used to be you could change anything layered on top of a map, but you couldn't change the underlying map itself. Now you have several options for changing the color palette and the visibility of different objects that make up your base map. The options vary on the amount you can customize and how much work is required. But all three of these APIs can have you map scripting with style.