Open Access (OA) literature is digital, online, free of charge, and free of most copyright and licensing restrictions. What makes it possible is the internet and the consent of the author or copyright-holder.
-- from A Very Brief Introduction to Open Access by Peter Suber
Open Access Factsheets
Open Access Resources
Resources for Authors
As in any business there are quality Open Access publishers and those with questionable motives and/or quality. Some sources to consider:
Open Access Journals
Open Access Textbooks / Books
Open Access Repositories
Open Access Theses
This is not an exhaustive list. Always check the copyright and permissions conditions before using material from any of the sites.