American Board of Clinical Neuropsychology
http://www.theabcn.org
American Board of Medical Specialties
http://www.abms.org
American Medical Association Physician Finder
http://www.ama-assn.org/aps/amahg.htm
American Society of Clinical Oncology
http://www.cancer.net/patient/Diagnosis+and+Treatment/Finding+Quality+Care/Find+an+Oncologist/Find+an+Oncologist+Database
Neurosurgeon Finder
http://www.neurosurgerytoday.org/findaneuro
National Library of Medicine - National Institutes of Health: Find a Doctor
http://www.nlm.nih.gov/medlineplus/directories.html
More Links
International Cancer Organizations
Pediatric Cancer and Brain Tumor Organizations