1) Gsnap. After lots and lots of testing, my personal favorite free pitch correction plugin is Gsnap. You can download two versions here:
http://www.gvst.co.uk/gsnap.htm (Newest Version)
https://www.dropbox.com/sh/gza2ayr4ylfuqe3/AADelAd4TdoDFr8UXQff-nUna?dl=0 (Older version)
2) Kerovee. My 2nd favorite free pitch correction plugin is called Kerovee. Kerovee can give your voice an awesome T-pain sound if you want. It can be downloaded at the following link:
3) Autotalent version 2. My 3rd favorite free pitch correction plugin is called Autotalent (version 2). Autotalent is extremely simple, yet surprisingly effective. Here's the link!
Click here to read my other post about how to add VST effects (.dll files) into Audacity