An SF company called Stamen Design does some interesting visualizations with OpenStreetMap data, including watercolor (http://maps.stamen.com/#watercolor/12/41.7656/-72.6756) and terrain. There's a nice post on how terrain and route number data is handled (http://content.stamen.com/terrain_process), with special acknowledgement to a guy you might have heard of named NE2 (http://www.openstreetmap.org/user/NE2).