These are freely available datasets from here and around the world. Some are accessible via API's, which are an automated way to mine data into software like Google Refine.
Data.gov.au -  Is hosted by the Australian Government. At present it contains over 1000 datasets, including datasets on Culture, and within that there is a photography dataset. Contains a bunch of interesting apps and links to GIS software also.
Datacatalogs.org  - Is a portal to open Datasets from around the world. Contains a couple of visualization datasets. API compatible.
The David Rumsey Historical Map Collection  - This is a database of 32,000 maps and images, focusing on rare 18th and 19th century North and South American maps, though the rest of the world is also represented. This collection can be viewed in conjunction with 2D and 3D GIS software as well as Google Earth and Google Maps.
Europeana  - Europeana is a single access point to millions of books, paintings, films, museum objects and archival records that have been digitised throughout Europe. It is an authoritative source of information coming from European cultural and scientific institutions.
Eurostat  - Business and demographic statistics from the Eurozone.
Statista  - A wonderful data portal with an easy to use search interface. It contains information on the art market, advertising including outdoor and billboard advertising, social media use and more.
World Bank Data Catalog  - A wealth of datasets from all parts of the globe. Contains World Development Indicators, Environmental stats, population forecasts and much much more! Largely API compatible