Black Cat Systems has released version 2.0.3 of Audio Toolbox
(http://www.blackcatsystems.com/software/audiotoolbox.html). This version fixes bugs that can cause problems when trying to play different waveforms at the same time.

Audio Toolbox is an audio function generator for the Mac. Using no
 additional hardware, you can produce sine, square, triangle, and
 sawtooth waveforms of any frequency up to 22 kHz, as well as generate
noise.

Audio Toolbox can be used to create all sorts of sound effects. You
 have full control over the frequency, type and volume of each sound
produced.

You can generate up to 13 different waveforms. Each waveform can be routed to either the left or right channel, or to both. The amplitude of each waveform may be adjusted, as well as the volume on each channel, which is a mix of any 
waveforms routed to that channel.
In addition, there's a sweep generator, Super, for producing "Whoop
Whoop" sounds, sirens, and more. Generated sounds can be saved as
WAVE recording files.

Finally, there are spectrum analyzer and oscilloscope windows. The 
spectrum windows displays the amplitudes of all frequencies of sound
present. A waterfall display is also available. The oscilloscope
window displays the waveforms of the signals coming into the left and 
right audio in-put channels.

Audio Toolbox versions are available for Mac OS X 10.5 ("Leopard")
 and later. Priced at $19.99, a copy of Audio Toolbox may be
 downloaded from the Audio Toolbox web site for evaluation before
 purchase.