Install the Unpaywall Chrome extension to automatically search the for OA versions of articles when you search on the open web.
"Open Access" (OA) is the practice of making research and writing free over the Internet. Authors, peer reviewers, scholarly societies and even commercial publishers participate.
The international movement began in the 1990s and grows stronger every year, as more researchers place their writing in journals and books available to the public for free. Open Access Overview by Peter Suber, Earlham College (and his Very Brief Introduction).
Librarians are major supporters of Open Access. We no longer have to use university dollars to purchase back research done by our own university's researchers published in commercial journals. The Association of Research Libraries created the SPARC organization to promote Open Access.
Open Access 101, from SPARC from Karen Rustad on Vimeo.
Here are individual OA journals and lists of OA journals.