Medscape's Physicians Connect and Sermo. Both are subsidized by some form of advertising, which is good since they would probably collapse if a fee were imposed on the users. I post on both, but have greatly curtailed my use of Physicians Connect which I think needs to eliminate anonymous posting and obtain a moderator. For an educated crowd, the physicians can be rather hateful, particularly those raised on revenge in the Middle East. I do not think a lot of them would want that level of candor identified with them and conveyed to their patients.
Politically, the most dedicated come across as libertarian: "Mine is mine and yours is yours, Average, though some say this is Sodom" as the sages of Pirke Avot observed. Sermo people have pseudonyms of their own choosing but not outright anonymity. Politeness is valued a good deal more on Sermo than at Physician Connect and remarks belittling another poster are rare, even on the political discussions, at the former though common enough at the latter that I took my intellect elsewhere.
People can also run medical dilemmas and opinions by specialists. While both contain a few medical loonies promoting their fringe agendas, the regular medical posters on each seem a reasoned bunch. Still, I get the sense that people who fundamentally like being doctors opt for Sermo, the kvetchers congregate at Physicians Connect.