MOBAC Map Evaluator is a unique, simple, small, Java based application specially designed to enable users to test and develop new map sources.
MOBAC Map Evaluator allows to interactively develop map sources that a more complex that the custom map format allows.
Usage is pretty simple:
Modify the code and press the "Execute code" button. If it does not work you can switch back to "Google Maps" or "OSM". If you use the reset button you can switch start new based on one of the three built in templates (OSM, Yahoo, Microsoft Bing). They show how a more complex example can look alike.