Stamen [http://maps.stamen.com/]
http://tile.stamen.com/toner/{z}/{x}/{y}.png
http://tile.stamen.com/terrain/{z}/{x}/{y}.jpg
http://tile.stamen.com/watercolor/{z}/{x}/{y}.jpg
http://tile.stamen.com/toner/{z}/{x}/{y}.png
http://tile.stamen.com/terrain/{z}/{x}/{y}.jpg
http://tile.stamen.com/watercolor/{z}/{x}/{y}.jpg
https://www.mapbox.com/help/how-web-maps-work/
http://mapschool.io/
http://www.liedman.net/tiled-maps/
http://leafletjs.com/examples/wms/wms.html
http://leafletjs.com/examples/zoom-levels/
https://en.wikipedia.org/wiki/World_Geodetic_System#WGS84
http://www.maptiler.org/google-maps-coordinates-tile-bounds-projection/
https://gist.github.com/tmcw/4954720
https://github.com/geometalab/pyGeoTile