Halfdan's Editor
A extensibility friendly Emacs-like editor written in Python using the PyGTK toolkit.
It is not meant to replace Emacs for _you_, but to replace (or at least complement) Emacs for me.
It tries to be as close to Emacs in the key bindings and generally does editing in the same way, you have a set of buffers that you move about by using arcane keyboard combinations which, for some in-explicable reason, makes you able to write really fast.
It also aims at being very easy to extend, actually i'm editing this page in my editor right now, using a SOAP web-service interface supplied by my homepage. Each time i save (C-x C-s combo), epihapny (that gnome browser you know) automatically updates the page. This functionality was achieved without touching any of the code within the editor, but only by editing the .he file, something like Emacs' .emacs file, only, .he contains Python code rather than Lisp.