Do a search for your topic using the Everything search box. Then on the left of the search results under Format, check Downloadable Article and Downloadable Archival Material. You'll get open access and institutional repository content from all over the world.
Less of a collection and more of a paradigm, CC licenses are more flexible than standard copyright and publishing licenses, many allowing free use without consigning the work to the public domain. Various searches are available through this site. Be sure to consult the exact text of the license before use.
Another search engine for Creative Commons licensed materials (images, audio, video, text, etc.)