My first commercial tool, FlexMotion is a code-driven play-on-demand animation system.
Instead of pre-configuring everything in the editor, FlexMotion allows you to play any animation when you want and how you need it to be played. All of this is done directly in the code which makes it a very efficient tool for prototyping. It's also a great fit for a project that implements its character state machine where Mecanim might be redundant.
There is nothing more validating than receiving positive feedback from your peers about your tools. For that, I will always be thankful for the tremendous effort I spent on FlexMotion.