Keirsey Temperament and Character Web Site - 
The Official Keirsey site. Take the Keirsey Temperament Sorter test online. Lots of articles, including several articles on Attention Deficit Disorder (ADD) and its relationship to Temperament.

http://www.keirsey.com/

Center for Applications of Psychological Type - 
Offers qualifying, certification, and advanced education training for the Myers-Briggs Type Indicator personality assessment tool. Publishes books, materials, and other instruments related to the MBTI and psychological type.

http://www.capt.org/

Association for Psychological Type - 
An organization focusing on the teaching and training of individuals using the MBTI.

http://www.aptcentral.org/

TypeLogic - 
Detailed profiles of the 16 psychological types. Supports a very active email discussion group on psych type

http://www.typelogic.com/

Personality Pathways - 
Articles and resources on applications of the MBTI, Myers-Briggs concepts, and psychological type to management, teamwork, leadership, communication, and personal growth and development