GNU emacs
GNU emacs is a popular emacs implementation.

When using GNU Emacs, it is advisable to put (setq lisp-indent-function 'common-lisp-indent-function) in your emacs init.el to enable common lisp style indentation. This depends on the cl-indent emacs lisp package which typically ships with emacs.

The project page is on http://www.gnu.org/software/emacs/emacs.html.