SparkNotes
This is a great review site or actually a site close to a complete book. It has a fairly detailed outline of US history, 45 question quizzes on each section (and there are many sections), context, important people, summary and analysis, study questions (including essays), and further reading. Really it is a one stop shop place for your students.

Another Quiz Maker
Now all we need is a test maker that teachers can access for free and then give their students. Short of that, this is another good one. http://www.triv.net/qmenu.htm is yet another one.

Online Quizzes
This four stemmed quiz bank allows a student to have as many as 35 questions per unit in US history and then gives the correct answers.