Schoolhouse Rock has a quick video that gives good information.
This site has lessons for all grade levels:
This site is for upper-grade classes. There are also some social studies games.
Site for students K-3:
Click HERE for coloring pages.
Here's an acrostic poem that you can download by clicking on the picture.
Here are some books you can share with your class:
Go check out some more freebie by clicking on the picture below: