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!
Free Stock Music offers royalty free music for registered users. There is a registration fee if you would like to have more music options, or you can join for free. Free registration will not provide as many songs, but with a little digging, there are some compositions that are licensed under Creative Commons.
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!
Public Domain Music offers released music from or before 1923. All of the music in this database is free for use and distribution!