Lending Club: Lending Club provides data about loan applications it has rejected as well as the performance of loans that it issued.
Medicare Hospital Quality: The Centers for Medicare & Medicaid Services maintains a database on quality of care at more than 4,000 Medicare-certified hospitals across the U.S.
Million Song Data Set: A freely-available collection of audio features and metadata for a million contemporary popular music tracks.
Pew Research Center: Pew provides data to the public for secondary analysis.
SEER Cancer Incidence: Data about cancer incidence, segmented by age, race, gender, year, and other factors.
Walmart: Walmart has released historical sales data for 45 stores located in different regions across the United States.
United States Census Data: The U.S. Census Bureau publishes reams of demographic data at the state, city, and even zip code level.
UNICEF: Data about the lives of children around the world including nutrition, immunization, and education.
Yelp: 6 million reviews spanning 189,000 businesses in 10 metropolitan areas