http://support.mapbox.com/kb/tilemill/terrain-visualization-with-digital-elevation-models-and-gdal
http://developmentseed.org/blog/2009/jul/30/using-open-source-tools-make-elevation-maps-afghanistan-and-pakistan
http://linfiniti.com/2010/12/a-workflow-for-creating-beautiful-relief-shaded-dems-using-gdal/