National Cancer Institute at the National Institutes of Health
Cancer.gov: Cancer Causes and Risk Factors
This site provides links to articles that deal specifically with many different causes of cancer, such as genetic, hormonal, or from a chemical that is inhaled or eaten.
PBS
Pbs Teachers: Cancer Warrior: How Cancer Grows
Follow the progression of a malignant tumor, beginning with the first mutation within a cell and ending with metastasis, the colonization of related tumors throughout the body.
CK-12 Foundation
Ck 12: Life Science: 11.56 Carcinogens and Cancer
Explore carcinogens and their relation to cancer.