Harmonizr is a vocal harmonizer offering a unique and fun user interface.  You can sing a melody, setting appropriate key centers, and Harmonizr will do the rest, generating up to 3 voices of automatic, perfectly-tuned harmony.  Edit your harmonizations and save them as presets, accessible via large, easy-to-use buttons.  

Connect to your favorite audio app by using Harmonizr as an AUv3 "music effect" plugin, or via Inter-App Audio.

Harmonizr also accepts MIDI input, either from an external hardware device, or from another app, which it can use either to set key centers, manually harmonize, or both.