Open access (OA) literature is a method of sharing scholarship that is "digital, online, free of charge, and free of most copyright and licensing restrictions. What makes OA possible is the internet and the consent of the author or copyright-holder. OA is entirely compatible with peer review, and all the major OA initiatives for scientific and scholarly literature insist on its importance. Just as authors of journal articles donate their labor, so do most journal editors and referees participating in peer review. OA literature is not free to produce, even if it is less expensive to produce than conventionally published literature."
--From Open Access Overview, by Peter Suber
Open Access is valuable because it:
Source - Coalition of Open Access Policies (COAPI)