One of the most exciting plug-ins I've tried lately is Antares' Harmony Engine. I don't know why, but I've always been attracted by vocal harmonizers. And since they're not a common thing in the plug-ins' world, I couldn't miss Harmony Engine!
It's a real time, "intelligent" harmonizer, and this makes it more fun and immediate to use it, especially for a creative/experimental approach.
At the same time, it's a very complex and flexible piece of software, and even if everything on the user interface is clearly laid out, I strongly suggest reading the manual (very well written) if you want to make the most out of thig plug-in.
Harmony Engine is a four-voices harmonizer, and each voice has several individual parameters (like pan, vibrato, voice character, etc.). Also, each voice can have its own audio output channel, so you can process the four voices in different ways.
Harmony Engine's real strenght lies in the Harmony Control modes: Fixed or Scale Intervals, Chord Degrees or Chord Name, Chord by MIDI, MIDI Omni, MIDI Channel.
In short it's everything you would expect from an intelligent harmonizer, and probably more! You can go from an 'automatic control' approach, to a 'note-by-note' manual one. There is much more to say about Harmony Engine, but since you can try it as a demo version (fully working), I would urge you to do that, it's much more gratifying than reading an article!
But don't expect magics from a plug-in: the quality and the intonation of the source material (both live and tracked) is essential. Then, as you will learn, there are many parameters, which with the right tweaks, can improve your final results.
Don't forget to check the demo songs that come with the installer, you can load them in your DAW, so you can see how engineers and artists have used Harmony Engine.
As said, I'm attracted by harmonizers more as a creative tools, and for that man-machine feeling you get from them.
Realistic results? Uhm, boring question to me, if you're a good singer just sing your part two or three times and you'll get a truly realistic result. But if you want a backup vocal that fits in the mix, Harmony Engine will do the job.
I have to say I'm really happy that something like Harmony Engine exists as a plug-in, so I can avoid spending money (and time) on hardware devices!
Platforms: OS X and Windows
Formats: VST, AU, RTAS plug-in.
Price: 349 $.
Read also the interview with Marco Alpert, from Antares.