Dance Vision International Dancers Association    http://dancevision.com

They are tops for instructional DVDs, tapes, and printed materials (but not inexpensive)!  I use the syllibi and support materials from DVIDA exclusively.  No other dance organization, in my opinion, provides the full spectrum of educational materials and resources for professional dance teachers and ballroom dancers who want to be the very best they can become.  My certification exams are provided by DVIDA, and by early 2008, I will have completed the Full Associate Certification for the 12 Syllabus Ballroom Dances.

Ballroomdancers.com   http://ballroomdancers.com

BallroomDancers.com provides a wonderful educational center for defining dances, for viewing brief demonstration videos, and even for finding song titles/artists to dance to.  You can order CDs there, too.

The National Council of Dance of America    http://ndca.org/

"The NDCA is the Governing Council of NDCA Sanctioned Events, recognized by the World Dance & Dance Sport Council as its sole representative in the United States of America."

"Its purpose is to provide, on a nation-wide basis, a united inter-association agency to represent the interests of those in the dance profession and other dance-related entities and organizations and to act as the agency for cooperation with similar councils in other countries."

"To also conduct a continuing campaign for the establishment and the maintenance of high standards in dance education, and to acquaint the public with the nature and benefits of these standards and to recognize the status of qualified dance teachers affiliated to member organizations of this Council."

USA Dance  http://usadance.org/

USA Dance is another national dance organization.  USA Dance's prime focus is promoting dance - first as a competitive sport and second as a social activity.  One of their goals is to get Dance sport into the Olympics.