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.
Your favorite museum not listed in the box to the right? Try these links: