auch - audio--checker

Here's an interesting FOSS app that describes itself as s a gnuplot-like program to visualize harmonics and test the effect of digital and analog filters...

auch - audio--checker

It might seem a bit on the esoteric side, but go take a look at the graphical representation of sound it produces. The Windows version has been out for a while now. The Mac OS X version (Tiger only) is labeled as experimental.