Tools

These four need no data and no network: they are arithmetic and a drawing, and they run in the page.

Transpose a key
Pick a key, pick how far to move, and the tool says where you land, what the interval is called and how many sharps or flats the new signature has.
Tempo and delay times
One beat is 60,000 ÷ BPM milliseconds. Every other number on this page is that one multiplied by a fraction, and the fraction is printed beside it.
Metronome
Set the tempo and the beats to a bar. The click is synthesised in the page; nothing is downloaded and nothing is recorded.
The circle of fifths
Keys are arranged by signature: each step clockwise adds a sharp. Pick a key and four light up — the relative, which shares its signature exactly, the two a fifth either side, which differ by one accidental, and the parallel, which keeps the tonic and swaps the mode. That is the whole of what a mixing wheel encodes, with the relations named instead of numbered.