Friture - Friture is a program designed to analyze audio input in real-time.

Audacity - Audacity is a free and open-source audio production software suite that includes a surprising array of editing tools and recording systems.

Visual Analyser - A powerful software implementing a Spectrum Analyzer, Oscilloscope, Frequency meter, Distorsiometer, Volt meter and more... plus complete D/A conversion, ZRLC, Impedance meter

Spek - Spek helps to analyse your audio files by showing their spectrogram.

xoscope - Xoscope uses the input of your Sound Card, EsounD and/or a ProbeScope/osziFOX and Bitscope-Hardware.

SoX - Sound eXchange is a command line utility that can convert various formats of computer audio files...