The “Pale Dawn” tileset has a calm, restrained style designed to work well with lots of data points. It’s less about the geographical information as such, and more about providing a suitable background to foreground information.
This map is constructed using a single image layer of CloudMade tiles. The tiles are in spherical mercator coordinates, and the map has standard interaction controls, so this basic example is a useful starting point for customization. Register a developer account with CloudMade for your own API key.