There are many good sites to obtain information on cancer, probably more than I have listed here. If you are aware of a site I should include, please let me know.
General Sites For Cancer Information
American Cancer Society - ACS is dedicated to helping persons who face cancer. Supports research, patient services, early detection, treatment and education.
Association of Cancer Online Resources - ACOR is a collection of online communities designed to provide timely and accurate information is a supportive environment.
Cancer Hope Network - To provide one-on-one support to people undergoing treatment for cancer, and to their families. We provide this support by training individuals who have recovered from cancer, and matching them with cancer patients currently undergoing a similar experience.
National Cancer Institute - The National Cancer Institute coordinates the National Cancer Program, which conducts and supports research, training, health information dissemination, and other programs with respect to the cause, diagnosis, prevention, and treatment of cancer, rehabilitation from cancer, and the continuing care of cancer patients and the families of cancer patients.
The National Lymphedema Network-A internationally recognized non-profit organization founded in 1988 to provide education and guidance.
OncoLink - Site with comprehensive infromation about specific types of cancer, treatments and research advances.
Cancer Specific Sites
Aplastic Anemia & MDS International Foundation, Inc. - The Aplastic Anemia & MDS International Foundation (AA&MDSIF) is the world's leading nonprofit health organization dedicated to supporting patients and families living with aplastic anemia, myelodysplastic syndromes (MDS), paroxysmal nocturnal hemoglobinuria (PNH), and related bone marrow failure diseases.
Breast Cancer - Breastcancer.org is a nonprofit organization dedicated to providing the most reliable, complete, and up-to-date information about breast cancer.
Dr. Susan Love, MD - The Dr. Susan Love Research Foundation works to eradicate breast cancer and improve the quality of women's health through research, education, and advocacy.  
Eyes on the Prize - A nonprofit corporation,providinginformation and emotional support from the survivors' perspective to women with gynecologic cancers, their families and friends, and healthcare providers.
Lung Cancer - A source of support and information for lung cancer patients and their families. Patients may also utilize CancerCare's free professional supportive services on their site.
Mesothelioma - This website has the most up to date and comprehensive information about Pleural Mesothelioma and asbestos exposure related cancer on the the web today. Such as Mesothelioma Survival Rate and information ranging from a complete list of symptoms, to treatment options, and steps to take after a diagnosis.
Mesothelioma Symptoms- The Mesothelioma Symptoms site covers a  range of topics from the different forms of mesothelioma, to the various methods of treatment.
Prostate Cancer Foundation - PCF source of support for prostate cancer research.
Skin Cancer - Leading skin cancer prevention organization.
Testicular Cancer Resource Network - An educational site for testicular cancer patients and their family.