Electro Harmonix Voice Box

This voice processor has nice features, but don't expect perfect harmonies, because there's not an option to assign a scale to the harmony. If you plug an instrument trough it, it is a bit more accurate though. Without an instrument providing the notes for reference, the octave harmonies are ok, but not so much with other intervals. It's not a total disaster, because it sounds quite musical and it has a lot of options to tweek and fine tune it to your needs.
I use it to thinken choruses and add some texture to vocals, and I am pleased with the results.