I used to hack a lot on Climacs, an Emacs-like editor written in Common Lisp. I still feel it is an interesting project: GNU Emacs has a lot of fundamental weaknesses and rapidly turns into a byzantine tower of complexity if you try to cover them up, partially due to the limitations of Emacs Lisp compared to more advanced languages.
I did not start the Climacs project, nor did I write all the current code, but I did rewrite major subsystems, like the redisplay engine (see my blog post for a description of what I did), which was not only fun, but very educational as well. My proudest feat was managing to create a syntax-highlighting redisplay engine that supported embedded images and variable-width fonts, and with somewhat acceptable performance, although in retrospect I should have spent more time studying the techniques employed by other editors, rather than just inventing the algorithms from scratch.
The best way to get Climacs is from its CVS repository, as described on the Climacs installation guide.