MEdit is a "programmable text editor driven by Tcl scripts", written by Maciej Sobczak.
The idea is to have a text editor that allows to define your own commands in the form of Tcl scripts that are executed in the editor's context. The functionality of the editor depends only on your ability to write in Tcl. There is a set of already written commands, to get you started.
It's particularly interesting for Tcl'ers because they can customize it quite easily. It runs on any platform with Tcl/Tk and is a ridiculous awful lot smaller than Emacs. Well, too many things are a ridiculous awful lot smaller than Emacs.
The home page of the project is http://www.maciejsobczak.com/prog/medit/