Wikimedia
Wikipedia: Gastrointestinal Tract
Wikipedia provides excellent resources and information on the gastrointestinal or digestive tract, including images and hyperlinked terms.
Mayo Clinic
Mayo Clinic: Colon Cancer
This site from the MayoClinic.com provides great information on Colorectal cancer, which includes both cancer of the colon and the rectum. The article includes facts on signs and symptoms, causes, risk factors, treatment and prevention.
Inner Body
Innerbody: Digestive System
Take a look inside the human body's digestive system. This site features diagrams of the digestive tract and each part is accompanied by labels and descriptions. Just select an organ by moving the mouse over its image.