As far as I know, there is no other editor with this capability. Sure, they all let you write "plug-ins" but none of them let you do your work by writing code interactively, alongside normal editing commands and GUI operations. As a programmer, this is how I want to do everything with my computer.
I don't have many customizations (I have something to move lines with the alt-key like Eclipse, and I swap parens and square bracket keys so it's faster to write Lisp) and I don't even know many keyboard shortcuts, but I write bits of Elisp all the time to perform large scale text manipulations, generate Java and html boilerplate at work etc. Having everything happen inside my editor, being able to use an editable buffer as an ad-hoc input source to a program just makes everything more fluid and interactive than doing the same thing with scripting languages on the command line.
It kind of baffles me that the Emacs people don't push this angle very much. There's hardly anything about it on the website. You'd think Emacs was just another editor with hundreds of keyboard shortcuts and a strange gui.