SOUNDS - Discover music with friends
Zorbi - Spaced-Repetition Flashcards - Other tools are built by boomers, Zorbi is built by students.
Spotify - Map shows when two people play same song at same time
Mochi - Write notes and flashcards with Markdown and study them with spaced repetition.
Songlink - Share music with anyone.
SmartCards+ - Spaced Repetition Software for iOS. Modern, powerful and easy to use.