If you are looking for pitch correction rather than harmony generation, take a look at Antares' AVP-1 rack unit. Same Auto-Tune technology used on most singers (including the greats) these days...

Use it reasonably subtly, and only on the out front sound (it'll mess with your mind if you use it on your monitors!), and it can help poor intonation quite a lot. It can't help poor tone or vibrato, but it can certainly help your pitch..!