Mathematics and Statistics

Government

• Canada Open Data, pilot project with many government and geospatial datasets.
• Chicago Data Portal, an open portal for government data for the city of Chicago. 
• Datacatalogs.org, open government data from US, EU, Canada, CKAN, and more.
• Data.gov.uk, publicly available data from UK
  • (also London datastore.)
• Data.gov, public access to datasets generated by the Executive Branch of the Federal Government.
  • Data.gov/Education, central guide for education data resources including high-value data sets, data visualization tools, resources for the classroom, applications created from open data and more.
• DataSF.org, a clearinghouse of datasets available from the City & County of San Francisco, CA.
• StatsPolicy, brings together important information about our interwoven Federal statistical system, including members, priorities, and activities across the Federal Government.
• National Government Statistical Web Sites, data, reports, statistical yearbooks, press releases, and more from about 70 websites, including countries from Africa, Europe, Asia, and Latin America.

Industry

• AWS (Amazon Web Services) Public Data Sets, a centralized repository of public data sets integrated into AWS cloud-based applications.
• Qlik (Subscription-based), visualize the world's economy, societies, nature, and industries, with 100 million time series from UN, World Bank, Eurostat and other important data providers.
• Google dataset search
• NASDAQ Data Store (subscription-based), provides access to market data.

Science and medicine

• Dryad is an open-access repository of research data, focused on scientific and medical research.
• GEO (GEO Gene Expression Omnibus), a gene expression/molecular abundance repository supporting MIAME compliant data submissions, and a curated, online resource for gene expression data browsing, query and retrieval.
• National Space Science Data Center (NSSDC), NASA data sets from planetary exploration, space and solar physics, life sciences, astrophysics, and more.
• NetworkRepository: Interactive Data Repository, has many collections of graph and networks from social science, machine learning, scientific computing, and other areas.

Culture

• Europeana Data, contains open metadata on 20 million texts, images, videos and sounds gathered by Europeana - the trusted and comprehensive resource for European cultural heritage content.
• Million Song Dataset, a freely-available collection of audio features and metadata for a million contemporary popular music tracks.
• Open Source Sports, provides a collaborative and inclusive environment that encourages everyone to contribute to and share the best rules for their sports.

Other

• GeoNames geographical database covers all countries and contains over eight million place names, which can be used to find geocode for countries, cities, suburbs, places and postcodes.
• KidsCount Data Center The Annie E. Casey Foundation's Kds Count (La infancia cuenta) is a premier source of data on children, youth and families. Find indicators for the nation, states and more.
• KONECT, the Koblenz Network Collection, with large network datasets of all types in order to perform research in the area of network mining.
• Re3data, a registry of research data repositories
• Stock Market Confidence Indices, formerly Robert Schiller's data on housing, stock market, and more from his book Irrational Exuberance..