Speccy

Speccy is a simple spectrum analyzer app. It's designed specifically for use in Amateur (Ham) Radio, where signals such as Morse Code are found at lower audio frequencies. Therefore, Speccy should display data between zero and around 6 kHz, which is actually still quite useful for other things, such as looking at the audio frequencies in your voice.

The top part of the app window shows the audio spectrum, and the bottom half shows those data over time in a waterfall graph. I wrote it as I use my Chromebook for listening to, and decoding, software defined radio outputs, so it's good to be able to see what the main frequencies are.

Also, if like me you use a variable bandpass filter when decoding signals (I use a SotaBeams module), then this app is very useful for seeing the effects of the filter and tuning the center frequency.