Five things you would otherwise open a second tab for
Reference pages that make a sound. A diagram of a chord tells you where to put your fingers; hearing it tells you what it is.
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Tuner
Chromatic, so it names whatever note it hears. Reads the microphone in the page and records nothing.
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Metronome
40 to 240 bpm with tap tempo and an accent on the downbeat, scheduled on the audio clock so it does not drift.
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Chord finder
Twelve roots and twelve chord types, lit up on a keyboard and playable, with the formula behind each one.
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Scale finder
Any scale from any root, played up and down, and an honest answer on which instruments can reach every note of it.
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Circle of fifths
Touch any key to hear its chord and its relative minor, and see why the order is what it is.
Open it
How these are built
Everything here runs in the page. There is no server doing the work, no account, and nothing to install. The tuner asks for your microphone because it cannot hear your instrument otherwise, and it reads the sound, shows a number and throws the sound away.
The chord and scale finders play through the same piano recordings the rest of the site uses, sampled from a real instrument and rendered through the app's own audio engine. The metronome makes its clicks with an oscillator, so it needs no samples and starts instantly.
Which one you want
If an instrument sounds wrong, start with the tuner. If your playing sounds wrong but the notes are right, it is usually time rather than pitch, and that is the metronome.
If you know the chord name and not the notes, the chord finder has it. If you are working out what will fit over a piece, or what your instrument is even capable of, use the scale finder: it answers whether a fixed-tuning instrument like a handpan or a guzheng can play the scale at all, which most scale tables never mention.
And if you want the reasoning rather than the answer, all of it is written up in the theory pages.