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.

Study Stack
So first off, some of my tech ideas come from this site which is amazing an just won an Edublog award this year. The "study stack" site has allows you to help your students prepare. I have struggled with study guides for years and think this may be better than what I have done in the past. I am going to have my students create their own accounts and define each of their own words and then use it to review (or make them do part of the review guide) and then do a partner's review. The cool thing is that it allows the students to input the data and then see it in a variety of forms such as flash cards, crossword puzzles, true false, etc. Here is the US history page.

Everything Site!
This site has it all:

Outlines
Important Documents
Political Parties
Practice Quizzes
Biographies
Court Cases
Political Timelines
Topic Outlines
Unit Notes
Vocabulary