Here's a wild stab... couldn't you use something like Auto Tune to create a harmonizer effect? (what?! hey!! stop throwing those tomatoes!!) I'm assuming you're looking for key/scale/mode intelligent pitch shifting, and AutoTune is the only plugin I know of that does it.
It'd sure be nice if Eventide made a plugin, but it'd probably cost 10 grand and require 4 dongles
Check google for shareware or freeware ones. I found one a few months ago that was great for church type sounding harmonies, (choral hall type thing). I have since formatted and not needed it since.
VSTs should be relativly easy to find.
I Like Clone Ensemble (adds up to a 32 part harmony). Steinberg's Voice Machine package is a really nice plugin too (Offers 2 plugins Voice Machine Processer, and Voice Machine Generator-offering MIDI note control). And of course Autotune makes a great harmonizer too (Offering by far, the most power of the plugins listed here).