America In Class: lesson plans, webinars and sources for students and teachers.
Digital History: primary sources, especially from underrepresented or oppressed groups throughout US history.
The Gilder Lehrman Institute of American History: primary sources and timelines for each era of American history with important events.
USHistory.org: two textbooks for students to peruse online, covering the span of history from Native American culture to present day.
Wikipedia and biography.com: Neither are considered reputable sites; though they can be quite useful when researching as a starting point to find references and other resources.