Fluid Designer is a Blender add-on that allows users to create and use parametric model libraries. Part of the project is an completely new user interface which has been entirely coded in Python. In this first development update, Andrew Peele demonstrates the extent of the modifications they applied to Blender's UI.
I have a LONG history with Blender - I wrote some of the earliest Blender tutorials, worked for Not a Number and helped run the crowdfunding campaign that open sourced Blender (the first one on the internet!). I founded BlenderNation in 2006 and have been editing it every single day since then ;-) I also run the Blender Artists forum and I'm Head of Community at Sketchfab.