BabyCenter is a resource on everything from pregnancy on up to tweens. Detailed information on each stage of development with milestones and helpful advice can be found here.
This website can help a parent identify what a child's illness is and if it is serious enough to see a doctor or will most likely resolve on its own and how to prepare a preschooler for the arrival of a new baby.
Community boards are available on Baby Center for support and information with parents who have children in similar stages or ages.