A program that monitors EsounD daemon and displays a graphical representation of sound in various gnarly and funn-key ways (ie, basic FFT and oscilloscope displays, and the new version also has more detailed spectrogram mode and such).

eXtace is part of GNOME, originally written by Rasterman - though I have to say eXtace isn't exactly GNOMEish in user interface. Well, intuitive nevertheless...