Data APIs, Hubs, Marketplaces, Platforms, and Search Engines

• AggData, aggregate data in 15+ categories, with a focus on location data, like Starbucks locations.
• Apertio, lets you search for millions of Open Data datasets.
• AWS (Amazon Web Services) Public Data Sets, provides a centralized repository of public data sets that can be seamlessly integrated into AWS cloud-based applications.
• AWS Data Exchange, with over 3,000 third-party data products (many free to use) from more than 200 data providers.
• Azure Data Marketplace (Microsoft)
• BigML Azure Data Marketplace widget
• CKAN, the world leading open-source data portal platform, aimed at data publishers (national and regional governments, companies and organizations) wanting to make their data open and available.
• data.com, part of Salesforce.com, source of D&B info and millions of contacts.
• Datacatalogs.org, open government data from US, EU, Canada, CKAN, and more.
• DataFerrett, a data mining tool that accesses and manipulates TheDataWeb, a collection of many on-line US Goverment datasets.
• Datafiniti, comprehensive & accurate product, business, and property data.
• DataHub.io, free data management platform from the Open Knowledge Foundation: publish, search, and get updates about data.
• DataMarket, visualize the world's economy, societies, nature, and industries, with 100 million time series from UN, World Bank, Eurostat and other important data providers.
• DataProvider, crawls the web to create a database of information about companies.
• DatastreamX, a global marketplace for commercial data, bringing together buyers and vendors of data onto one simple-to-use platform.
• dataX, a collection of datasets curated and crowdsourced by CrowdAnalytix community of data scientists
• Enigma Public, the world's broadest collection of public data.
• Exversion, search 100K+ datasets, consume them through one simple API or upload your own data to collaborate, publish, or share.
• Factual location platform enriches mobile location signals with definitive global data, enabling personalized and contextually relevant mobile experiences.
• Figshare, a platform for researchers: figures, datasets, media, papers, and presentations can be published in a citable, searchable, and shareable way.
• Freebase, a community-curated database of well-known people, places, and things.
• GDELT: The Global Data on Events, Location and Tone, described by Guardian as "a big data history of life, the universe and everything."
• GetData.io, Public data sources maintained by your community.
• Google Public Data, with dynamic visualization and exploration tools.
• Handshake, an app and a website that allows you to negotiate a price for your personal data directly with the companies that want to buy it.
• InfoChimps, Data Marketplace and Big Data Platform
• Knoema, provides statistical profiles, commodity prices, and other data for countries around the world.
• Marinexplore.org, a platform to explore, discover, and share public ocean data.
• Mode Analytics, a collaborative, easy-to-use platform for working with data, SQL code, visualizations, and results.
• National Government Statistical Web Sites, data, reports, statistical yearbooks, press releases, and more from about 70 web sites, including countries from Africa, Europe, Asia, and Latin America.
• Open Data Census, assesses the state of open data around the world.
• OpenData from Socrata, access to over 10,000 datasets including business, education, government, and fun.
• OpenDataSoft, a cloud-based platform for seamless and unlimited data re-use.
• OSDC: Open Science Data Cloud Repository, for very large public data sets of scientific interest.
• Plotly, data and visualization platform, with tens thousands of datasets in science, academia, public policy, and business.
• Ready Signal time series data, with hundreds of curated control data sets including economic, weather, demographic & health. Easily output data at desired time period and geographic granularity, and apply/test 15+ Data Science Treatments.
• Tableau Public, free software for visualizing and sharing data
• Quandl, a collaboratively curated portal to millions of financial and economic time-series datasets.
• qunb, a platform to find and visualize quantitative data.
• Visa Data, the power of the Visa Network delivered as an API.
• Zanran, search the web for data and statistics.

Other collections:

• Big Data Startups Public Data platforms.
• BigML Data Marketplaces and Data Hubs bundle and BigML list of thousands of Public Data Sources

Related

• Government, Federal, State, City, Local and public data sites and portals