Isabel Healthcare uses the symptoms you enter to suggest a list of medical diagnosis possibilities. While it does not provide medical advice, from the list you can read a Wikipedia article on each topic to narrow it down. For a limited time a version is free to individuals with no account needed. It covers about 6000 conditions.
Isabel was designed by a dad with the intensive care doctors who treated his daughter Isabel during a health crisis. The system can help consumers “make sense of the vast amounts of medical knowledge out there, become better informed and work constructively with your doctor to find the right diagnosis as soon as possible.”
Although no account setup is required, a help manual is linked on the registration page.