Erlang the runtime sure. But Emacs is an application with these properties. There is no editor like Emacs for Erlang (some small clones I am sure) and no other user application utilizing this feature.

Smalltalk is a development environment and an applications platform. The development tools are as extensible and user customizable as Emacs. And like Erlang it is also a runtime you can package software for. Any GUI or server software. For Emacs you can probably do the same, but I’m not convince many users would find the GUI familiar.