Digital Public Library of America is a platform that hosts millions of digitized projects from the United States' universities, archives, museums, and more. Many of the works that have been aggregated through DPLA are under creative commons licenses, or exist in the public domain.
Flickr offers Creative Commons images when searching through its database of user-uploaded images. These range from CC BY Attribution to CC BY-NC-ND as well as Public Domain options.
Free Music Archive offers music from current composers that have placed Created Commons licenses on their music. Some of the composers have even gone as far as placing their music in the Public Domain!
Creative Commons: Music Communities: A list of different music communities that licenses music under creative commons licenses.
Jamendo offers royalty-free music licenses and hosts a great community for musicians and music lovers alike.
NYPL Digital Collections offers over 650,000 items and growing everyday. These items are free for all to view, and although some of them are in the public domain, some images do have restrictions that will need their permissions requested. Their database offers drawings, rare and illuminated manuscripts, maps, photographs, videos, and more.
Opsound is a community of musicians who share their music that you may use freely for commercial and noncommercial use. They license all of their music under Creative Commons and encourage creativity from the users.
Project Gutenberg offers over 59,000 ebooks that can be downloaded in various formats. There is no fee, and they're all free!