Gojimo, http://www.gojimo.com/, is a free revision app which is available on the App Store and Google Play store that allows you to quickly revise specific topics from a variety of subjects.
The app allows the user to select a specific qualification level, for example GCSE or A level, after which they have to select a subject from the ones listed and finally select which examining board they are doing. The app doesn’t only include UK qualifications but also some international ones. This is all completely free and overall it has to offer over 40,000 questions.
The great advantage of this app is that it doesn’t require much working out since the questions aren’t too complex and they are multiple choice, but at the same time it does a great job at helping you revise and explains to you how to get to the answer. It also keeps progress of how well you have done in each topic, meaning that you can see what you are worst and best at.
Sometimes you might need to use a pen and paper to get to the answer which you might not have available if you are revising on the bus or train, but i think that the app is fantastic overall for revising and keeping track of the topics you need to revise further and all of it is for free so check it out!