In principle there are two options available for scientist to make their publications freely available:
The Green Road: free accessibility via the secondary publication (Self-Archiving). Meanwhile, many publishers of traditional toll access journals have a policy in place allowing the authors of the articles published in their journals to make the pre-/postprint and in some cases even the publisher's version of the articles freely available via the website or the institutional repository of the institution with which the author is affiliated. This policy might include an embargo period. The use of this option does not involve any charges! The Max Planck Digital Library (MPDL) is operating PuRe the central institutional repository of the Max Planck Society.
The Golden Road: free accessibility of the primary publication.This is achieved by either publishing an article in a genuine Open Access journal or a hybrid journal. In so-called hybrid journals only some articles are freely available whereas genuine Open Access journals have a policy to allow free access to all of their articles.
Frequently, in both cases a publication fee is charged from the author.
Detailed information on INTRANET