This site has a wide range of materials - presidential communications, election data, speeches, party platforms etc... There are audio and video clips from late 1800s to the present. The PBS webpage on The Presidents is geared toward teaching the history of the American presidency. Biographies provide links to primary documents.
This is a wonderful resource constructed by the Yale law School. More than 3500 full text documents on US legal, economic, and political history. The documents are divided into 5 time periods.
Another excellent site from the American Memory collection. You can search records, debates, bills, and proceedings of America's national legislative bodies beginning with the 1774 Continental Congress and continuing through the 42nd Congress in 1873. Special presentations include "Indian Land Cessions in the United States, 1784 to 1894" and "The Impeachment Trial of President Andrew Johnson."
The Constitution of the United States of America: Analysis and Interpretation (popularly known as the Constitution Annotated) contains legal analysis and interpretation of the United States Constitution, based primarily on Supreme Court case law. This regularly updated resource is especially useful when researching the constitutional implications of a specific issue or topic. The Featured Topics and Cases page highlights recent U.S. Supreme Court decisions that demonstrate pivotal interpretations of the Constitution's provisions.
Developed by the Legal Information Institute at the Cornell Law School, this site allows you to easily search Supreme Court decisions in a variety of different ways. You can also search for opinions by state courts and federal courts.
A central site to find local, state, and national government information. For specific information about the different branches of government look at The White House, House of Representatives, Senate, and the Supreme Court.