HathiTrust is a partnership of academic institutions that provides access to digital books. Particularly useful for access to older public domain texts and materials created by its member institutions. Note that not all materials listed in this catalog have full-text access.
Project Gutenberg, founded by Michael Hart, is an online repository of books that have entered the public domain in the United States. This source is particularly useful in finding older works of literature, philosophy, etc.
Flickr, a photo posting and sharing website, hosts this collection of photos in the public domain or with Creative Commons licenses, allowing other users to use these photos according to the license the creator has chosen.
TED (Technology, Entertainment and Design) specializes in making talks and presentations about different topics available to the general public. These can be particularly useful with respect to topics about innovation, culture, and technology. Note that some speakers are there to promote themselves or their businesses.
This website, founded by Wikimedia, provides access to open images, sounds, and videos created by users. Note that, while some items are public domain, others are licensed through Creative Commons and thus require attribution. (For more information about Creative Commons licenses, go here.)