Sonic Visualiser is an open source application for viewing and analysing audio files.
At first glance, the program looks like many other audio tools. Open an audio file (MP3, OGG or WAV), a standard waveform is displayed, and you can scroll through it by clicking "Play".
You don't have to live with a single waveform, though. Sonic Visualiser can add more, perhaps giving separate views for the left, right and combined channels. There are spectrogram, melodic range, peak frequency and spectrum views, again based on your preferred channels, and you can display and combine as many as you need.
Each pane is exceptionally configurable: there are multiple channel views (Separate, Mean, Butterfly), scale options (dB, Linear, Meter), gain, pan and balance tweaks, amongst others.
This means you're able to view the same audio data in many different ways. Click "Play" and each pane displays its own custom view on the current track. Support for time-stretch playback (slowing down or speeding up while retaining a synchronised display) also helps you examine areas of interest.
Sonic Visualiser can then annotate your files by adding labelled time points, segments, point values and curves, and optionally overlaying these later.
And if this isn't enough, the program can be extended further with assorted free Vamp plugins. As we write, these include beat trackers; tempo and key estimators; pitch, tempo and note trackers; harmony and chord extraction; timbral and rhythmic similarity estimators, and plenty more.