Harmonizr

Harmonizr is a vocal harmonizer offering a unique and fun user interface.  Plug in a pair of headphones and sing a melody, tapping the correct key center buttons, and Harmonizr will follow, synthesizing up to three voices of automatic harmony in your own voice.  Plug in a MIDI keyboard, or use the on-screen one, and you can sing harmony like Imogen Heap or Jacob Collier, matching every note you play.  Harmonizr also can also be used as an Audio Unit (AUV3) extension within other apps, such as GarageBand, AUM, or Cubasis, and can be controlled by MIDI controller apps like SoundPrism.

Version 1.9 release notes

Apologies, a compiler issue caused v 1.8 to not actually harmonize. Kind of the core functionality, and I didn't catch it before distributing. Thank you for your patience!