Digestive Disorders Foundation - UK charity with a good range of online information, including constipation, diarrhoea, irritable bowel disease, liver disease, colitis, gallstones and cancer.
IBD Club - Information for sufferers of inflammatory bowel disease (ulcerative colitis and Crohn's disease), written by Addenbrookes Hospital.
Crohn's Disease in the UK - An informal site established by a sufferer from Chrohn's disease, with information, links and a message board.