How to get there: http://my.webmd.com/chat_rooms
Rating (out of 5): download - 5 (if using JavaLight, no download is required);  features - 4; chatters/population - 3
Requirements: Sign up for an account with WebMD.  Provide email address and zip code - optional newsletters.
Download:  Use Java Light for basic chat or download iChat plug-in for advanced features.  Some users can only use html (yuck).  Java Light is adequate for first visits.
Special Note: The first visit is a little confusing. After you sign up for an account, there is no easy way back to the chatrooms - use the URL above or wind your way through the Member Communities links.

Room Selection: All medically related - from ADD to mental health to pregnancy. Most are support group, and may or may not have a WebMD host. 

General Comments:

• The list of chat rooms also includes the number of chatters currently signed into it. If there is a chat room with a topic you *really* want to discuss, go ahead and enter the chat room, then wait for others to join you. Someone has to be first :-)
• Live events - A list of scheduled chat, with a WebMD host and designated topic, are available.
• Bulletin Boards are also available on many health related topics, and appear to be extremely active
• WebMD is an excellent resource for anything health related.  Customize your  WebMD for your own needs/interests.