Doctor Finder, an online service of the American Medical Association (AMA), provides information on the education, training, and certifications of physicians throughout the United States. It is not required that physicians be members of the AMA in order to be included, but physicians are required to fill out paperwork sent to them on a regular basis by the AMA. If they do not complete it, they are not included either in the online or book version (the AMA Directory). If a physician's name does not come up in a search, it does not mean (s)he is not a physician; it may just mean that (s)he did not fill out the forms.
Who's Certified? is a service of the American Board of Medical Specialists. It provides the names and board certification information only for specialists who are certified by the ABMS. Who's Certified? requires a registration process and restricts users to five searches per day. Don't expect to find anything here but the name of the board that certified the physician.
Most states have online search engines through their medical society, department of health, or state licensure agency. Sometimes, these services search ALL licensed professions (including nurses, pharmacists, and non-medical professions). NOAH has a listing of some of these Websites. The amount of information available (name, license number, educational background, complaints registered) varies from state to state.
Many professional societies also have search engines that will permit you to look for a specialist who is a member of their society. Some examples include: American Society of Clinical Oncologists, American Academy of Neurology, American Association of Clinical Endocrinologists, American Gastroenterological Association, and American Academy of Allergy, Immunology and Asthma.
Castle Connolly publishes the series "Best Doctors." They have mounted a Web version, a section of which is available at no charge (though it is required that you register for the service). You can search an abbreviated version of the "Best Doctors" book, as well as a variety of magazine articles that rate "Best Doctors".