http://bicyclesafe.com/ This site is a goldmine of useful safety advice for bicyclists, and an especially good page on preventing collisions with vehicles.
http://www.bicyclinginfo.org/ focuses on prevention, and community promotion of bicycle safety.
http://www.dmv.ca.gov/about/bicycle.htm The California DMV’s page for bicycle safety and bicycle law
http://www.nhtsa.dot.gov/portal/site/nhtsa/menuitem.810acaee50c651189ca8e410dba046a0/ The National Highway Traffic Safety Administration’s page on bicycle safety programs
http://www.helmets.org/guide.htm The Bicycle Helmet Safety Institute’s guide for buying the right helmet
http://kidshealth.org/kid/watch/out/bike_safety.html A good page on bicycle safety for children