Does anyone know any sources for free European city boundary data?
The EU has a Corine landcover derivative dataset, Urban morphological zones, which gives polygons of urban areas across Europe defined as:
A set of urban areas laying less than 200m apart
The link to download is: http://www.eea.europa.eu/data-and-maps/data/urban-morphological-zones-2000-umz2000-f1v0-1
The page also contains detailed information about how the data was generated and the data is provided as ESRI shapefiles.