U.S. Access Board - This agency wrote the Americans with Disabilities Act Accessibility Guidelines(ADAAG).
U.S. Department of Justice, Office of Civil Rights - This agency adopts regulations and enforces the ADA.
U.S. Housing and Urban Development (HUD) - This agency wrote the Fair Housing Amendment Act (FHAA) and enforces its implementation.
The Southwest ADA Center, based in Houston, provides research, education, consultation and training in areas of Independent Living Research Utilization.
Below is a direct link and brief description of each agency.