Web/Android/iOS: GoConqr gives students and teachers a place to upload and organise class notes, flash cards, mind maps, quizzes and more. Whether you're a student using it to learn or a teacher using it to distribute materials to a class, there are tools to help you find what you want to know.
GoConqr is actually the latest incarnation of previously mentioned ExamTime, which, aside from a new name, has a whole new look and mobile apps for iOS and Android to help you study on the go. The service is similar feature-wise to what it's always been -- you can still add your own notes, build mind maps to connect topics, make flash cards for things you need to memorise, and save old quizzes. You can also make your notes public so the rest of the community can use them, or keep them private so only you can see them.
The service is still completely free to use, although free accounts come with a few limitations (free users can only make their notes public, and they can only upload notes for three "subjects".) You can check out their different plans here. The mobile apps are free and free to use. Hit the links below to try them out or learn more.