The different layers seem to be defined in [this bit of code][1] (see LayerCode lines). So if you look at the layers available in the layer switched switcher in the top right of the main site, M is the default Mapnik rendering, C is cyclemap, T is transport map and Q is Mapquest Open. [1]: http://git.openstreetmap.org/rails.git/blob/HEAD:/app/assets/javascripts/map.js.erb

The different layers seem to be defined in [this bit of code][1] (see LayerCode lines). So if you look at the layers available in the layer switched in the top right of the main site, M is the default Mapnik rendering, C is cyclemap, T is transport map and Q is Mapquest Open. [1]: http://git.openstreetmap.org/rails.git/blob/HEAD:/app/assets/javascripts/map.js.erb