City mayors: statistics
City Mayors is a leading international source for urban statistics.
Cities
DataPortals.org
Directory of open data catalogs form around the world. Includes statistics and GIS products for different geographical levels (countries, regions, cities, etc.).
State of the cities data systems (SOCDS)
The SOCDS provides data for individual Metropolitan Areas, Central Cities, and Suburbs. Includes statistics on demographic and economic characteristics of the population, employment, crime, building permits, government finances, and principal cities.