It listens to the microphone, finds the pitch with the YIN algorithm, and shows a band of stripes turning against the nearest note:

Stripes standing still — the note is in tune.
Drifting right — you are sharp, tune down.
Drifting left — you are flat, tune up.
The drift speed follows the error, so the last few cents crawl — which is the whole point of a strobe display.

Two views: a strobe disc of six concentric octave rings, and linear bands that magnify the final cents. A4 calibration (415–466 Hz), the noise gate and the in-tune window are all adjustable.

In the browser

Nothing to install: allow the microphone and play a note. The audio is processed entirely on your device and never leaves it. Works on a phone too.

Download

Windows 64-bit. For the VST3, unzip it, copy the Strobe Tuner.vst3 folder into C:\Program Files\Common Files\VST3 and rescan plugins in your DAW. The files are unsigned, so Windows SmartScreen may warn you: More info → Run anyway.

The app and the plugin are built with JUCE under JUCE’s AGPLv3 option, which makes both of them AGPLv3. That licence gives everyone who receives a binary the right to the corresponding source under the same terms, which is why the source package is up there too. JUCE itself is not in the package; where to get it and how to build are in the package’s own README.