Mac Audio Toolbox is an audio function generator for the Macintosh. 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.
Mac Audio Toolbox can be used to create all sorts of cool sound effects. You have full control over the frequency, type and volume of each sound produced.

You can generate up two ten 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 is a sweep generator. Super for producing "Whoop Whoop" sounds, sirens, and more.