These days there is an AUv3 for just about anything, and also pretty much anything you could possibly want. Now, along comes
Harmonizr, which is a vocal harmonizer offering a unique and fun user interface, if you believe what the developer has to say.

Apparently 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.

All of which is great, but, in my experience harmonisers aren’t always very human sounding. Having said that, I’m willing to give it a try, and, strangely, just the other day I found myself working on a track where I thought the vocals could use some harmony. Coincidence? Maybe…

Anyway, Harmonizr is on the app store and costs $4.99