Mainstream GNU Emacs and derivates:
Other implementations still alive:
Other more or less obscure implementations:
Not locally documented:
It is a pretty long page, though.
This document, by CraigFinseth, provides a partial – and not comprehensive – list of implementations of Emacs-type editors and literature about such editors. You can help make it more comprehensive by sending the author additional information and/or updates.
This document is also gradually acquiring the role of a repository of (at least) Emacs-related history. While that subject requires a book to itself, this document will probably serve until someone (not the author!) writes one.
See also: EditorsUsedByOthers