emacs-cl, Emacs Common Lisp (this link is dead, here is an archived version) is a Common Lisp implementation written in Emacs Lisp. That means you can run Common Lisp programs in Emacs, and you can write Emacs extensions in Common Lisp. At this stage, much of ANSI Common Lisp is implemented, but, among other things, CLOS and pretty printing are missing.

2009-02-11 Links to CVS repository, tarballs and mailing list from the homepage are currently broken