Data.gov http://data.gov The US Government pledged last year to make all government data available freely online. This site is the first stage and acts as a portal to all sorts of amazing information on everything from climate to crime.
US Census Bureau http://www.census.gov/data.html A wealth of information on the lives of US citizens covering population data, geographic data and education.
Socrata is another interesting place to explore government-related data, with some visualisation tools built-in.
Data.gov.uk http://data.gov.uk/ Data from the UK Government, including the British National Bibliography – metadata on all UK books and publications since 1950.
Canada Open Data is a pilot project with many government and geospatial datasets.
Datacatalogs.org offers open government data from US, EU, Canada, CKAN, and more.
Healthdata.gov https://www.healthdata.gov/ 125 years of US healthcare data including claim-level Medicare data, epidemiology and population statistics.
UNICEF offers statistics on the situation of women and children worldwide.
Huge resource of public data, including the 1000 Genome Project, an attempt to build the most comprehensive database of human genetic information and NASA ’s database of satellite imagery of Earth.
Facebook FB +0.65% Graph https://developers.facebook.com/docs/graph-api Although much of the information on users’ Facebook profile is private, a lot isn’t – Facebook provide the Graph API as a way of querying the huge amount of information that its users are happy to share with the world (or can’t hide because they haven’t worked out how the privacy settings work).
Face.com: A fascinating tool for facial recognition data.
UCLA makes some of the data from its courses public.
Google Public data explorer includes data from world development indicators, OECD, and human development indicators, mostly related to economics data and the world.
Junar is a data scraping service that also includes data feeds.
Buzzdata is a social data sharing service that allows you to upload your own data and connect with others who are uploading their data.
Gapminder http://www.gapminder.org/data/ Compilation of data from sources including the World Health Organization and World Bank covering economic, medical and social statistics from around the world.
DBPedia http://wiki.dbpedia.org Wikipedia is comprised of millions of pieces of data, structured and unstructured on every subject under the sun. DBPedia is an ambitious project to catalogue and create a public, freely distributable database allowing anyone to analyze this data.
Freebase http://www.freebase.com/ A community-compiled database of structured data about people, places and things, with over 45 million entries.