Public Library of Science (PLoS)

PLoS publishes a number of open-access biological and medical journals, which are rapidly becoming premier journals in their respective areas.

From the website: "PLoS is a nonprofit organization of scientists and physicians committed to making the world's scientific and medical literature a freely available public resource." "Everything we publish is freely available online for you to read, download, copy, distribute, and use (with attribution) any way you wish."