I believe it's a vocoder, fed with some polyphonic (possibly even a bit noisy) carrier + the vocal, and mixed with the dry vocal signal at the end.

Look into Elastique Pitch. They have a newer version Elastique Pitch 2, but I only have the old one. Super easy to use. You can go from chipmunks to deep low voice. I have found that it works best when changing a female vocal to deep male.

Also people have mentioned Melodyne.