I released a Chrome extension called Audio Scope. It shows you the audio playing in any tab: a triggered oscilloscope, a spectrum analyzer, and an X-Y mode for music composed to be drawn on a screen.
My dad sold oscilloscopes for Tektronix when I was growing up, so the instrument was familiar long before I understood it. Building one for the browser was a way to experiment with real-time visualization and to spend more time with audio.
Browser FX: