OpenStreetMap
Routing is a very good way to highlight errors in OSM data. I have begun a process to identify and correct roundabouts in my area of Oxfordshire, England whose directional arrows point anti-clockwise instead of clockwise, because I have found so many to be incorrect.
Discussion
Comment from Pierre Mauduit on 20 January 2009 at 21:55
When I began mapping, I found a roundabout in the wrong way near my hometown in France. Then I wondered "mmmh, there must has been either a drunken driver or an english mapper by there ..." ;-)