As an avid fan a peer reviewed journals, I get very excited when a journal does not charge a fee for its content. A new medical journal has recently been introduced, Open Medicine, from Canada. Its authors are the martyr type, having recently been fired from another open source journal over issues of editorial freedom. I wish them well in this new endeavor.
The Directory of Open Access Journals provides a very comprehensive listing of open journals, not just from the medical field, but from a variety of scientific disciplines. Of course, you can always conduct your search queries in the Pub Med Central engine, where all results are going to be free public domain.