• American Association for the Advancement of Science (120,000+)
  
• American Association of Petroleum Geologists
  (a somewhat equivocal statement in 2007)
  
• American Association of Wildlife Veterinarians
• American Astronomical Society
• American Chemical Society
• American College of Preventive Medicine
• American Geophysical Union (50,000+)
  
• American Institute of Physics
• American Medical Association
• American Meteorological Association (12,000+) (Also this)
  
• American Public Health Association
• American Quaternary Association
• American Society for Microbiology
• Geological Society of America (21,000+)
  
• National Academy of Sciences
• National Academies Press
  
• Society of American Foresters

And "Skeptical Science" commentary on national organizations around the world