Panoramio is a Google-owned service where anyone can upload photos of places where they have been to.
Unlike Flickr and other photo sharing websites, photos uploaded to Panoramio need to be approved by the community and only pictures that illustrate places are added to the site. For instance, if you capture a picture of the Taj Mahal, it makes a good candidate for Panoramio but if your friends are also posing in that same picture, the community is less likely to approve it.
Also see: 3D Tourist Maps of Important Cities
Panoramio is therefore a decent, if not perfect, way to measure the relatively popularity of different regions /cities of the world as far as tourism is concerned. The higher the picture count from a region, the more popular that place should be among tourists.
Using a similar logic, Ahti Heinla has created an interesting heat map on Google Maps that shows how popular different parts of the world are among tourists. The underlying data for the heat map is sourced entirely from Panoramio.
The color yellow on the world map indicates high density of tourists (most of Europe), red indicates medium activity (see Japan) while blue means that destination is less popular with tourists. Places that had no Panoramio photos, and therefore nil tourist activity, are in grey color. [via]