Audacity - Audacity is a free and open-source audio production software suite that includes a surprising array of editing tools and recording systems.
Friture - Friture is a program designed to analyze audio input in real-time.
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...
Signalizer - Signalizer is an all-in-one signal visualizing package for VST, VST3 and AudioUnits