Many within the GIS community working on global data are familiar with the Gridded Population of the World (GPW). This global gridded population estimate has now been more available through a web-service. This lets the user zoom around to investigate the data without having to download the rasterdata.
Additionally and more interesting are the population estimation service, that can be accessed through OGC, REST and SOAP interface to estimate the population within a drawn polygon.
One of the new mapping tools also released, based on the technology used by Google Maps, demonstrates the Population Estimation Service. It lets users select an area of interest by drawing a polygon on the map and submit the request to the service, and it displays the results. (CIESIN News)
The google map based Population Estimation Service can be accessed here.