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.