If you set the "Retune Speed" to a very slow setting (e.g., 0.2 seconds) and the "Threshold" low, you can smooth out a shaky vibrato without turning the vocalist into a robot. I recorded a demo of "Hallelujah" where the chorus was drifting sharp. A light pass of the default correction made it listenable—not perfect, but listenable.
When it comes to free audio editing, Audacity is the undisputed king. It is the first port of call for podcasters, musicians, and voice-over artists working on a budget. However, one of the most searched queries regarding the software is how to make it "Auto-Tune" a voice.
: For over a decade, "how to autotune in Audacity" has been a staple search for aspiring bedroom producers, involving a specific "rite of passage": manually downloading a DLL file and moving it into Audacity's hidden plugin folder. How the Plugin Works Today