There are several freeware automatic pitch-correction plugins and I've tried them all. By far, the one with the most arrestingly good effect is KeroVee's. It's the best-sounding and its GUI is arguably the best-looking. It makes lovely R&B warbles, strong pop-vocals, the characteristic T-Payne effect, absolutely all of that good stuff one would want from an automatic pitch-correction plugin. I whole-heartedly recommend this to anyone looking to manipulate or correct vocals.

(For bonus fun, try sending all your reverb to a track with this on it. It results in something that sounds like a bit like a glitchy string section.).