The American Dental Hygienists Association provides an Oral Health Overview, including easy instructions for proper brushing techniques and a whole lot more.
http://www.adha.org/oralhealth/index.html
The American Association of Orthodontists provides a wealth of information about braces, orthodontics and the many benefits of a beautiful smile.
http://www.braces.org/
The American Academy of Orofacial Pain outlines comprehensive information about temporomandibular dysfunction (TMD or TMJ) including TMD symptoms, anatomy and treatment.
http://www.aaop.org/
The American Dental Association is a great resource for all types of dental questions.
http://www.ada.org/