FourTrack Audio Recorder/Mixer for iPhone
Sonoma Wire Works, the people responsible for the popular recording software RiffWorks, have developed an app for the iPhone and iPod Touch which allows users to record and mix their own audio with up to four tracks.
Intended as a cursory recording tool – a drawing board of sorts – the FourTrack app has no way to view waveforms while recording, but it does allow you to listen to previously-recorded tracks while recording a current one.
The iPhone’s speaker is obviously not professional grade and the audio gets recorded as 44.1 kHz .wav files, but as far as being compared to humming a tune in your head until you get to the studio goes, FourTrack’s got quite an edge. Get it here from iTunes.