iComposer is a new app for the iPhone/iPod touch that's designed to turn your hummed melodies into MIDI data that can then be played back by one of 130+ built-in instruments.
The premise is that you can capture your moments of inspiration whenever or wherever they strike (though humming into your iPhone when you're on the bus might mark you out as something of an oddball).
Once your score has been generated, it can be edited in the staff editor, and it's possible to play back up to eight different parts simultaneously. The bank of supplied sounds includes piano, organ, drum, string, guitar, reed and brass instruments.
Songs can be exported as MIDI and compressed audio files, and you can set key and time signatures.
You can learn more about iComposer on the developer's website. This contains examples of the audio-to-MIDI conversion in action.
The software costs $8.99/£5.49 and is available now from the Apple App Store.