I believe I saw GNUmed missing on the Open Source medical software list. GNUmed is available free and open source for MS Windows, GNU/Linux and MacOSX. We are physicians from German, France, Canada, Australia and Spain and are looking into extending it to be usable in a variety of health sytems. For some reasons there is no developers from the US so we don't have any billing so far. I believe it can be added easily.
Have an nice day.
http://wiki.gnumed.de