Most museums have a tab dedicated to educational resources on their sites. Teacher resources are often divided by grade level so you have quick access to age appropriate material.
American Memory - invaluable resources for teachers & how to use resources in the classroom. Also has a "today's date in history" section; Much historic video, audio, and photos.
Smithsonian - left menu has links to separate sections for kids, another for teachers. Also tabs for Exhibitions and Research.