Colonial Williamsburg Official History Site extensive site for this topic along with games and information for teachers..
Social Studies Alive is an interactive site which tests a student's knowledge of various aspects of United States History.
The Columbus Navigation Homepage has everything for a study of Christopher Columbus you may be able to use some information for younger children.
Have Fun with History has a small but nice selection of various aspects of United States history videos and activities.
USA Printables contains coloring pages for American symbols, great for notebooking or unit/theme studies.
Documents and Symbols of American Freedom offers lesson plans for grades 3, 5 and 8. Extensive links on people, documents, symbols and more.
USPS Community Connection is a great site to study history in a different way, free resources I would say for elementary.