Many websites offer free tests. Some textbook publishers and educational companies offer standardized tests and detailed explanations for the correct answers with the answer keys.
The CollegeBoard provides official SAT Practice tests online and as downloads. It includes questions and answers, reviews and study tips. Many of these tests are previous SAT questions, so students can be prepared for the SAT specifically.
The Princeton Review has a wide array of tests available for people to use, including the ACT, the SAT, AP exams and the LSAT. As with CollegeBoard, many of these tests questions are from previous versions, which helps students learn how the questions are worded.