Gracias a Bernard Marr, que ha realizado esta muy buena y útil recopilación, para muchos de los que trabajáis con datos abiertos, big data, visualización, periodismo de datos, etc...
Data.govhttp://data.gov
US Census Bureauhttp://www.census.gov/data.html
European Union Open Data Portalhttp://open-data.europa.eu/en/data/
Data.gov.ukhttp://data.gov.uk/
The CIA World Factbookhttps://www.cia.gov/library/publications/the-world-factbook/
Healthdata.govhttps://www.healthdata.gov/
NHS Health and Social Care InformationCentrehttp://www.hscic.gov.uk/home
Amazon Web Services public datasetshttp://aws.amazon.com/datasets
Facebook Graph https://developers.facebook.com/docs/graph-api
Gapminderhttp://www.gapminder.org/data/
Google Trendshttp://www.google.com/trends/explore
Google Financehttps://www.google.com/finance
Google Books Ngrams http://storage.googleapis.com/books/ngrams/books/datasetsv2.html
National Climatic Data Centerhttp://www.ncdc.noaa.gov/data-access/quick-links#loc-clim
DBPediahttp://wiki.dbpedia.org
Topsyhttp://topsy.com/
Likebuttonhttp://likebutton.com/
New York Timeshttp://developer.nytimes.com/docs
Freebasehttp://www.freebase.com/
Million Song Data Sethttp://aws.amazon.com/datasets/6468931156960467