BasicDSP is an educational tool that makes it easy to experiment with simple
Digital Signal Processing algorithms for audio signals. The input can either
be taken from the sound card, or be a locally generated sine wave, white noise
or impulse signal. The output is fed to the sound card, as well as to a virtual
oscilloscope and spectrum analyzer. 

WWW: http://wwwhome.cs.utwente.nl/~ptdeboer/ham/basicdsp/