ACT Reading Practice TestsOfficial ACT Website Quizzes 50-75 questions in all subjects, 6 Sample Essays. It makes sense to use the ACT's own test preparation for everything it's worth. The questions are fairly comparable to actual ACT tests. The questions are broken up into groups of 10 - 15, so it's easy to take one and return later. The answer explanations are very in-depth, and the sample essays are useful. Yahoo 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.
ACE Critical Reading Practice Tests 20 Critical reading tests of about 15 - 20 questions. This is a superb resource if you want dedicated critical reading practice tests. There are a ton of questions and it tells you if you got a question right or wrong. Unfortunately, there are no explanations for questions you miss, but the site is still worth checking out.
Study My English Reading Comprehension 38 Reading questions. All tests are available in both PDF and Word format. No answers are online, but you can always ask someone to help you if you need any on a few particular questions.
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.
SAT Reading Tests can also be used as supplementary material since they are fairly similar.