Back when I was a grad student, I was a devoted Emacs user. I basically used it like an operating system: it wasn't just my text editor, but also my mail reader, my Web browser, my news reader, and so much more. (I once even asked our sysadmin to change my default shell to /usr/bin/emacs. He refused.) So when I started doing development in the S language, it was inevitable for me to think I could tweak some existing Emacs scripts and make it easier for me to edit S code. Sure enough, this turned into a major project (S-mode) with several collaborators, that culminated in being able to run the S interpreter within Emacs, and get (then-radical) features like command history and transcript management. When R came along, a new team adapted S-mode for R, and ESS -- Emacs Speaks Statistics -- was born.
I'm ashamed to say that while I still use Emacs occasionally, I never got around to installing it on my Mac. Today, in the ESS tutorial at useR! presented by Stephen Eglen I learned that I needn't have hesitated, because pre-compiled binaries of Emacs for Windows and Mac are available thanks to Vincent Goulet, which include ESS (and much more) pre-installed. No configuration necessary, just install and run -- I was up and running with ESS in less than 2 minutes.
If you're not familiar with ESS and want to try it out yourself, go ahead and download Emacs bundled with ESS, and check out this introduction to ESS from the CrossValidated blog, or download Stephen's tutorial slides (which also include a detailed introduction to Emacs, if you're not already familiar with the basics).