I use the TC Helicon Voice Works Plus, it's a rack mounted version (not a pedal).
They take a bit of experimenting and tweaking too get them right, but when you get it to fit properly in a song it works great.
I use it primarily with one band that does everything from old school rock & funk to all the new hip hop stuff that I just don't understand
The person you intend on using it with (yourself, or someone else) needs to have a good solid voice, not just your friends telling you that you have a good voice.
Because when you sing off key, now you have 2-3 part harmonies or vocal doubling singing off key.
I like them, they do take some tweaking, they can be a F'n feedback machine if your not carefull, but I am happy I bought one.