MacDSP is currently under development, but you can download a copy of the latest release, and give it a try.
Your comments and suggestions, as well as bug reports, are certainly appreciated. The latest version of MacDSP breaks the audio spectrum up into ten bands and the amplitude of each band can be controlled. It works much like an equalizer does in your stereo. Right now, the bands are linearly spaced at 2 kHz intervals. Ideally, they should probably be spaced on an octave or similar basis. How does it work? You connect your audio source to the Mac's microphone jack, select the low and high pass frequencies, and MacDSP will do the rest.
Please read the documentation for more information.
MacDSP is only available for PowerPC equipped Macintosh systems.