Robert, the creator of ZipScribbleMaps [1][2][3], has added a dozen countries from the geonames postal code data set to his collection of ZipMaps : AT, AU, CA, CH, DE, ES, FR, HU, IT, NL, NO, SE.
ZipScribbleMaps is visualising the organisation of postal codes by connecting postal codes in ascending order and using different colours for different administrative divisions.
Robert has spottet a couple of errors in the geonames data set during this work. Montejo de Bricia for example was erroneously laying in the Bay of Biscay.
GeoNames Blog 
Nice stuff! I just made similar map of Finland:
White background: http://www.ihminen.org/latitude/postinumerot.php?valkoinen
Map background: http://www.ihminen.org/latitude/postinumerot.php
It’s slow, for it uses javascript and google maps.