PLoS is a non-profit publisher. All of their journals are open-access and are paid for by publication fees. They have only been around for a few years, but in that time, their top-line journals (PLoS Biology, Comp Bio, Medicine, Genetics, etc...) have become top-tier class journals (one notch below Science, Nature and Cell, depending on who you talk to).
They are no lightweight in the biomedical research journal landscape.
They are no lightweight in the biomedical research journal landscape.