A metronome that keeps honest time. Set the tempo, or tap it in if you know the feel but not the number.
Why this one does not drift
Most browser metronomes run off the ordinary page timer, which is fine for a few bars and then slowly slides as the browser gets busy with other work. This one schedules each click against the audio clock instead, which is the same clock the sound card uses. Over a long practice session that is the difference between a metronome and a suggestion.
Practising with it
The usual advice holds: set it slower than feels necessary, play the passage cleanly four times in a row, then move up by four or five beats per minute. If you cannot play it cleanly at the slow tempo, playing it faster only makes the mistake permanent.
The first beat of each bar is pitched higher so you can hear where the bar starts without counting.