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, including 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.

Eurostat - Business and demographic statistics from the Eurozone.

Statista - A wonderful data portal with an easy to use search interface. Try searching for anything you like - 'internet shopping', 'US Elections', 'social media' and see what it can do!

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