Audio Scope
Audio Scope is an iPhone application that uses the remote i/o audio unit in the iPhone for handling audio input and output.
The application can display the input audio in one of the following forms: a regular time domain waveform, a frequency domain waveform (computed by performing a Fast Fourier Transform on the incoming signal), and a sonogram view (a view displaying the frequency content of a signal over time, with the color signaling relative power, the y axis being frequency and the x as time).