A great list of where your child should be at developmentally from 1 month to 17 years old.

A great resource of pdf files of what you can look for to see if your child is developing and what you can do to help.

Communicating with your kid at every developmental level - great site.

PBS list of milestones physically in the first five years.

PBS list of social/emotional milestones in the first five years.

PBS list of thinking milestones in the first five years.

A whole website dedicated to how kids develop and what skills they have when.