Here are some links to government agencies that publish information about toxics:
Agency for Toxic Substances and Disease Registry (ATSDR)
Publishes lengthy Toxicological Profiles for many different toxic chemicals. Summaries of the profiles called ToxFAQs are also available.
Environmental Protection Agency (EPA)
Maintains the Integrated Risk Information System, which has data on the health effects of many individual chemicals.
Maintains a list of Chemicals and Pollutants, with links to additional information.
International Agency for Research on Cancer (IARC)
Publishes Monographs on various known or suspected cancer-causing agents. See also their summary of Agents Classified.
National Institute for Occupational Safety and Health (NIOSH)
Focused on workplace exposures. They publish the NIOSH Pocket Guide to Chemical Hazards.