The acronym of the Public Library of Science.
A nonprofit publisher, which operates on an open-access basis.
From the Wikipedia page
The Public Library of Science began in 2000 with an online petition initiative by Nobel Prize winner Harold Varmus, formerly director of the National Institutes of Health and at that time director of Memorial Sloan–Kettering Cancer Center; Patrick O. Brown, a biochemist at Stanford University; and Michael Eisen, a computational biologist at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory. The petition called for all scientists to pledge that from September 2001 they would discontinue submission of papers to journals that did not make the full text of their papers available to all, free and unfettered, either immediately or after a delay of no more than 6 months. Although tens of thousands signed the petition, most did not act upon its terms; and in August 2001, Brown and Eisen announced they would start their own non-profit publishing operation. In December 2002, the Gordon and Betty Moore Foundation awarded PLOS a $9 million grant, which it followed in May 2006 with a $1 million grant to help PLOS achieve financial sustainability and launch new free-access biomedical journals.