i maintain a lingering fondness for the [aurora text editor](http://www-personal.umich.edu/~knassen/aurora.html), a dos-based text editor that was, like emacs, largely written in its own scripting language. the editor was sadly closed-source and has now gone the way of most closed source stuff, but the scripted bits were open source, and they were a real pleasure to work with. again, like emacs, you could build some surprisingly non-text-editor-like things on top of it.