'ESS (The Emacs Speaks Statistics) (ESS) mode provides a tight integration of the powerful GNU R statistics system into Emacs. In addition, ESS has support for SAS, BUGS (Bayesian Uniform Gibbs Sampler) and JAGS (Just another Gibbs Sampler). Finally, it also provides an equally tight integration with S-plus, direct descendant of the S language created by John Chambers at Bell Labs. It comes with an editing mode for .R files providing syntax highlighting as well as an interactive R console based on the Emacs comint mode.
ESS also provides (along with AucTex) a superior editing environment for noweb files, (typically files with an Rnw extension). These files mix R code and LaTeX text and markup and can be used to facilitate reproducible research. Important commands for Sweave mode are M-n-s, which runs all the code-blocks in a Rnw file, M-n-w which copies the code chunk at point, and M-n-k, which kills the code chunk at point.
ESS also provides an interface to the R help system. Help for any topic can be accessed using C-c C-v and typing the name of a function or operator. A new buffer containing the source code for the function at point can be created using C-c C-d.