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 Planning, Property, Transport and Culture. Contains a bunch of interesting apps also.
Datacatalogs.org  - Is a portal to open Datasets from around the world. API compatible.
Eurostat  - Business and demographic statistics from the Eurozone. Also contains specialist datasets like rural development.
World Bank Data Catalog  - A wealth of datasets from all parts of the globe. Contains World Development Indicators, World Bank Finances, Health, Education Stats and much much more! Largely API compatible.