Digital Public Library of America
The Digital Public Library of America (DPLA) aggregates cultural heritage content digitized by local partners. DPLA Pro links you resources for developers and sample code for easily downloading and working with DPLA content. 

HathiTrust
HathiTrust offers access to millions of books digitized from libraries around the world.

HBLL Digital Collections
The Harold B. Lee Library has digitized thousands of historic photographs, newspapers, diaries, and books. 

Internet Archive
The Internet Archive provides access to millions of digitized books, music, and software. 

Project Gutenberg
Project Gutenberg features more than 50,000 digitized books. 

Registry of Research Data Repositories 
Search or browse the Registry for data sets in multiple academic disciplines.

Additional Sources lists on the Data research guide.