ACT Science Practice TestsYahoo Education ACT Test Prep 10 questions in each subject. Even though there aren't too many questions, Yahoo's questions are of good quality and have very thorough explanations. The site is easy to use and it grades questions for you. Explanations are given after each question.
Barron's Free ACT Quizzes 20 questions. While there's not a whole lot of questions, there is a timer included in the online quiz. Explanations are given for incorrect questions, and an approximated score is given. Registration is short, and there is no need to give a real name or email if you don't want to. You can try Barron's if you want a good 20 minutes of learning and aren't expecting much more.
Study Guide Zone 10 Math, 5 Science, 10 English, 10 Reading questions. The questions here are limited in number, and the explanations are short. There is a pretty good study guide if you want something other than what the ACT website provides.