Autotune isn't free, but there's a free alternative called GSnap. (Included with Mixcraft as of version 5)
These programs can also be used to correct the pitch of a vocal that is slightly out of key and Autotune is often used in commercial songs to avoid having to do another take when a small part of vocal isn't perfect the first time around.
To use them in Mixcraft, place the DLL in C:\Program Files\VST\ or go into Mixcraft's Effects preferences and add the plugin's folder to the list of VST folders Mixcraft is searching.
Note: there are Youtube videos on how to tweak these plugins to get the sound you want.