Quizlet is a free online learning tool that is also available as a free mobile app for iOS and Android devices. Quizlet is especially helpful for learning vocabulary, as it allows you to either create your own study set or use study sets already created by other users. Users can quiz themselves by using the study sets like traditional flash cards, or choose from the other study modes (Speller, Learn, Test, Scatter, and Space Race).
inClass is a free iPad/iPhone/iPod Touch app that not only allows you to take text, audio, and photo notes, and to share notes with your classmates, but it also helps you stay organized by keeping track of your class schedule and reminding you of assignment due dates.
OpenStudy is a social learning network where students from all over the world can ask questions, answer questions, and connect with other students studying the same thing. OpenStudy integrates with Facebook and is also available as a mobile application.