The Circle of Fifths Calculator is an interactive wheel: tap any major key on the outside or its relative minor on the inside. You get the key signature spelled out, the scale with playback, the relative, parallel, dominant, subdominant and tritone keys, every diatonic triad and 7th chord with Roman numerals and functions, and six popular progressions written out in that key with play buttons.
It is the fastest way to see how keys relate, to find the chords that belong in a key, and to understand why neighbouring keys on the circle share so many notes.