In addition to being a good source of health information on a variety of topics, the Mayo Clinic provides this useful tool to better understand possible causes of symptoms that you may be experiencing. Of course, your doctor is best equipped to make a true diagnosis.
This website is run by the American Academy of Family Physicians and offers another symptom checking tool in flow chart form.
This is an interactive symptom-checker for parents interested in learning more about symptoms that their children may be experiencing. This site is run by the American Academy of Pediatrics.