American Academy of Periodontology
http://www.perio.org/
This professional association has a wealth of information about periodontics, especially the causes and dental treatment of gum disease, an excellent section on dental implants and a thorough overview of cosmetic dentistry. Before and after photographs provide powerful illustrations of results.
American Dental Association
http://www.ada.org/
An authoritative resource for a broad range of oral health topics including consumer information for the general public, educational references, research updates and general news about dentistry.
Michigan Dental Association
http://www.smilemichigan.com/
A superb collection of public information, especially the section Oral Health A-Z. Also provides valuable information about evaluating dental insurance coverage plans.