Pitch Correction (Auto-Tune)

Pitch Correction is the process of correcting the intonation of an audio signal without affecting other aspects of its sound. Pitch correction first detects the pitch of an audio signal (using a live pitch detection algorithm), then calculates the desired change and modifies the audio signal accordingly.
There are two popular pitch correctors used in the professional world of audio; Antares "Auto-Tune" and Celemony "Melodyne". According to wikipedia Auto-Tune has versions for Linux, however I was unable to find them. Melodyne is Windows and Mac, and they provide a demo version of their software which I have played with before on Windows and it's fantastic, with a graphical way to easily shift notes and create harmonies, also now with the ability to pitch correct single notes in a chord of notes (a feature which I think Auto-Tune has in some of their devices).
Unfortunately there are no Linux versions for either of these programs (none which I can find) and even if their were; they are quite expensive. So let's do it the free open source way.