Australian polygons - http://www.abs.gov.au/AUSSTATS/abs@.nsf/DetailsPage/2033.0.55.0012011?OpenDocument - Download the KMZ for Postal Area IRSD, SEIFA 2011. Unzip and upload the kmz
Coverage: AUS
Geometry type: polygon
Canadian polygons - http://www12.statcan.gc.ca/census-recensement/2011/geo/bound-limit/bound-limit-2011-eng.cfm - Download ESRI Shp, Forward Sortation Areas, Digital Boundary
Coverage: CAN
Geometry type: polygon
USA polygons - http://www2.census.gov/geo/tiger/TIGER2013/ZCTA5/tl_2013_us_zcta510.zip
Coverage: USA
Geometry type: polygon
French polygons - http://www.data.gouv.fr/dataset/fond-de-carte-des-codes-postaux
Coverage: FRA
Geometry type: polygon
CartoDB's got geometries from:
Worthy of an investigation?