I'm not a singer (rhythm guitar is my usual job) but singing in harmony i can get away with it, just. But we had a gig lined up (well, singing a few songs at the kid's school summer fair...)and the other singer in our band was unable to make it...So i did a bit of digging on the internet and saw a few vocal stomp boxes with all sorts of bells and whistles and price tags to match. Too much money for what might just be a one-off gig or at best, limited use.
Then i found the TC Helicon VoiceTone H1 Intelligent Harmony Pedal. It has three knobs, does one job; which is to add harmony to to a single vocal.
I must admit to being a little sceptical that it would be much good, because it was so cheap (relatively) and that it would be able to track my voice in key as i played my guitar through it. I'd seen a few reviews of more expensive boxes that said it is important to sing in tune!
But it worked really well! Not just playing chords through it, but it managed to track a finger-picked mic'd banjo as well... It certainly had some of the more observant members of the audience trying to figure out where the harmony was coming from and it was great fun to use. A robust construction too, so i see no reason why it should nto last for years.
A couple of times the pitch waivered (probably down to me not singing the note in key or playing the wrong chord) but the audience didn't seem to notice and it added a little humanisation to it to the harmony which was no bad thing. I only used it the "guitar" mode and did not try just setting it one particular key but as this should be easier for it i have no reason to think this would not work as well. Also, when the harmony was set to be lower than me it wandered a little when i was singing low myself, but that again was probably due to inaccuracy from my voice in the lower range...
Like a lot things, less is more and it might get tedious if you used it all through every song but it is dead easy to switch on and off as needed i.e. between verse and chrous, so that was not an issue.
All in all, a simple and brilliant piece of kit either for the solo vocalist or for a band where one of the singers has taken the day off.... Highly recommended!