ACT Math Practice TestsTest Prep Review's Self-Assessment Modules 400 Math questions, 50 in other subjects. This is a good site for practicing your ACT Math skills. The math questions are broken up by topics with around 20 questions per topic. There is an answer key included, but no explanations are given. There are also online lessons where you can brush up on topics you feel weaker on. It's a solid math testing service, but it's lack of explanations limits its use in learning from your mistakes.
4Tests Practice Quizzes 25 English, 30 Math, 20 Reading, 20 Science questions. These questions are written by Barron's, but they are online free here. You can see answers to questions as you take them, or view them all at the end. The site grades your quiz for you. While there are explanations, they can be short and are not always informative. Regardless, it's a great site for practicing and learning.
Number2 ACT Prep 40 Math questions, 20 in all other subjects. These questions are given individually without a timer. If you answer a question incorrectly, the site gives you a hint and sends you back for another guess. Once you answer it correctly, the site gives you an explanation. Their vocabulary drill is very useful and well-constructed. You can review new words, words you previously missed, or words you've mastered. All words are used in a sentence after you correctly answer them. There is a short registration for Number2, but it's worth the vocabulary tool and quizzes.
Algebra.com Quizzes 50 Math questions, 16 Vocabulary questions. While the site is for both SAT and ACT review, the math questions are very similar to actual ACT questions. You can choose to see the correct solution at the end of each problem, or at the end of the whole quiz. Explanations are very effective, since they are linked to related lessons if you feel a need to brush up on any topic. Many words are linked to the Webster dictionary, which is very useful on the Vocabulary quizzes, but is of small importance on most math questions. There is a very fun site for vocabulary tests here. Algebra offers great math help that's easy to use and offers a unique approach to learning through testing.
Math Tests There's not too many questions here, but there are explanations and are useful nonetheless.
SAT Math Tests can also be used if you run out of ACT Math quizzes to take.