American Cancer Society
American Cancer Society: Overview: Breast Cancer
This excellent site covers all the issues related to breast cancer. Topics include the causes of breast cancer, risk factors, prevention, early detection, diagnosis, staging, treatment, questions for your doctor, after treatment, and so on.
Aetna Intelihealth
Aetna: Inteli Health: Breast Cancer
Thorough overview of breast cancer. Includes information on symptoms, treatment, prevention, and much more.