Tweener is a tool similar to TweenMachine or aTools/animBot. It allows you to quickly create in-betweens or adjust existing keys by interpolating towards adjacent keyframes, and can speed-up your workflow when creating breakdowns and in-betweens.

I am an animator myself and so I very much tailor the tool to my own needs.

Get it from my GitHub https://github.com/mortenblaa/maya-tweener