From Celemony Software:
Melodyne is a programm that allows a completely new approach to the handling of audio material. It analyzes the pitch and time of monophonic audio files (from, for example, singers, wind or string instruments) and offers the opportunity to change whole melodies in a way only previously possible at MIDI-Level. Melodyne is able to change the musical parameters of voices or instruments without any actual influence on the character of the recording. Melodyne extracts the pitch and the rhythm, and provides an area in which to play with the audio material. Any change in this material is interpreted by the software in a musically intelligent manner and the result will always sound natural. Working with Melodyne is as easy as editing notes with MIDI.