Blender is very much the Emacs equivalent for 3D editing software. It had a kernel of functionality written in C but the majority of the functionality is actually written in Python, which means there is really robust support for extensions and plugins.

Also like Emacs, the UI changes based on what mode you are currently in, which is super confusing at first because it is often not clear which mode you need to be in to accomplish some operation.

Also like Emacs, they have managed to cram an amazing amount of functionality into the program so it seems to do everything, and is rock solid (it would never crash, when 3DS Max would crash 3-4 times a day).

It's not for everyone, but it is very powerful.