The Social Science Research Network and the Public Library of Science host the most extensive, publicly accessible collection of research papers. While PLOS is a library of open access journals within science and medicine, SSRN contains working papers on disciplines within the social sciences.
SSRN contains more than 500,000 full text research papers, organized in 23 research networks. Any user can register and submit her papers to the network, provided that the author has the permission of the copyright holder (if not herself) to post the work.
PLOS is a collection of peer-reviewed, open access journals. Authors may submit manuscripts to any PLOS journal, but the paper has to undergo a peer-review process before publication. A group of other scientists within the same field read the paper and submit a review to the editor of the journal, who in turn decides whether to publish the research paper based on the reviewers' evaluation.