You can play melodies or harmonies using a MIDI controller, instantly re-pitching your voice to match any note. Legitimate "Free" Options & Alternatives
If the price tag is a bit high right now, here are the best ways to get that Polyverse magic without breaking the bank: Buy Manipulator | Pitch Shifter - Plugin Boutique
Developed in collaboration with the legendary electronic duo Infected Mushroom, Manipulator uses unique granular algorithms to alter the timbre and pitch of monophonic audio.
Includes pitch and formant shifting, harmonics, sound smearing, and more.