Crash Course is a series of 15 to 20 minute YouTube videos by John Greene. When you click on the link, scroll down to the courses "U.S. History" and "U.S. History & Government." The "U.S. History" link focuses more on the actual history, while "U.S. History & Government" summarizes the Constitution and how the government functions.
He talks REALLY fast, at the same time, he does a great job at zeroing in on details and gives you extra information useful for essay writing. Check it out!