There are vast numbers of CC-licensed resources available on the web, but there is no central registry for this material, so they can be tricky to find. The Creative Commons website does include a search page with links to some of the largest collections of licensed materials, as well as a directory of CC-licensed content, (which is not, however, always up to date). These are good places to start. However, as none of these searches are fool-proof, be sure to verify that the results are indeed under a CC license. If in doubt, check with the author.
Note: Always check the terms of use! CC license terms can vary slightly in different jurisdictions, and different sites have differing requirements for proper attribution, etc.
Europeana "is a single access point to millions of books, paintings, films, museum objects and archival records that have been digitised throughout Europe. It is an authoritative source of information coming from European cultural and scientific institutions.site created by universities, museums, and other cultural institutions in Europe, which includes text, images, and audio/visual materials." To find CC-licensed or public domain materials, you can filter your initial search results by "copyright" and get only freely-usable materials.
USA.Gov is a portal to the vast collections of the U.S. federal agencies, including text publications, videos, images, and maps, almost all in the public domain.
Wikisource is an online library of free content text publications, in several languages. Unless otherwise noted, all content is released under a CC BY-SA license.
The World Bank, "the largest single source of development knowledge," has created the Open Knowledge Repository to make their books, World Development Reports, working papers, journal articles, etc., freely available on the web. The OKR currently contains thousands of works, and more are being added daily, almost all under a CC BY 3.0 license
The Biodiversity Heritage Library is a partnership between a consortium of natural history and botanical libraries and the Internet Archive to digitize and make freely available historic titles from their collections. These include works in the public domain, as well as works still under copyright by special arrangement with publishers.
The Medical Heritage Library provides free and open access to historical materials in medicine from the collections of some of the world's largest medical libraries. Most of the materials are in the public domain, but some copyrighted materials are also available by special permission of the publishers.
Open Education:
The Open Educational Resources Commons is a portal to freely available learning materials. These include open courseware sites (MIT OpenCourseWare, Kahn Academy, Peer 2 Peer University), and open textbook projects (Flat World Knowledge, Open Textbooks, College Open Textbooks). All material at the OER Commons can be reused, revised, and redistributed.
Open Science:
The Science Commons is a Creative Commons portal to open access initiatives in science. Although the project was apparently terminated in 2009, the links go to ongoing projects.
The OpenScience Project is an initiative to write open source scientific software.
Other interesting open science projects are referenced in the WikiPedia article, "Open Science."
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