Gojimo is available as a free app for Apple and Android devices as well as a web based version that works with any device.
Gojimo has over 160,000 free quiz questions on review topics for all major exams including ACT, SAT, and AP. All questions onGojimo come from quality sources such as Oxford University Press and McGraw-Hill Education. Students can create a free account and their review progress will be automatically synced across devices, great for using at home and at school.
Here is an example ACT Math question using the web version:
The right side of each question gives an explanation to help students understand the correct response. Each quiz consists of five questions in a particular domain or you can select a random quiz. The interface is very simple and easy to understand.