The Crohn's and Colitis Foundation of America (CCFA)
This site provides information about Crohn’s and colitis disorders, events, programs and research in America.
http://www.ccfa.org/

American Gastroenterological Association (AGA)
http://www.gastro.org

American Society of Gastrointestinal Endoscopy   (ASGE)
http://www.asge.org/

American College of Gastroenterology (ACG)
http://www.acg.gi.org/

American Liver Foundation
The American Liver Foundation (ALF) is the nation's leading nonprofit organization promoting liver health and disease prevention.
http://www.liverfoundation.org/

Celiac Disease Support page
Celiac Disease Gluten-free Diet Support Center at Celiac.com provides important resources and information for people on gluten-free diets due to celiac disease, gluten intolerance, dermatitis herpetiformis, wheat allergy, or other health reasons
http://www.celiac.com

International Foundation for Functional Gastrointestinal Disorders
This site provides information on irritable bowel syndrome, fecal incontinence, bloating, abdominal cramping, gas, and indigestion.
http://www.iffgd.org/