Most of my students will know the importance of the Cycle of Fifths several lessons into their guitar studies with me.
Its great for practicing the guitar, practicing improvisation, composing/songwriting and understanding how keys, modes, chords and their progressions work. If you write your own songs you'll find the Cycle of Fifths 'wheel' a very useful thing indeed :-)
Now software developer, Rand Scullard, has come up with a truly interactive online version of this important music tool and you can find a free copy here: Rand Scullard - Cycle of Fifths - complete with a detailed 'User Guide'.
Thanks to Carlos at Schwilly Family Musicians for sharing this find.