The latest development version of ESS is available via https://svn.R-project.org/ESS/, the ESS Subversion repository.  If you have a Subversion client (see http://subversion.tigris.org/), you can download the sources using:

     % svn checkout https://svn.r-project.org/ESS/trunk PATH

which will put the ESS files into directory PATH.  Later, within that directory, `svn update' will bring that directory up to date.  Windows-based tools such as TortoiseSVN are also available for downloading the files.  Alternatively, you can browse the sources with a web browser at: ESS SVN site (https://svn.r-project.org/ESS/trunk).  However, please use a subversion client instead to minimize the load when retrieving.

If you remove other versions of ESS from your emacs load-path, you can then use the development version by adding the following to .emacs:

     (load "/path/to/ess-svn/lisp/ess-site.el")

Note that https is required, and that the SSL certificate for the Subversion server of the R project is

     Certificate information:
      - Hostname: svn.r-project.org
      - Valid: from Jul 16 08:10:01 2004 GMT until Jul 14 08:10:01 2014 GMT
      - Issuer: Department of Mathematics, ETH Zurich, Zurich, Switzerland, CH
      - Fingerprint: c9:5d:eb:f9:f2:56:d1:04:ba:44:61:f8:64:6b:d9:33:3f:93:6e:ad

(currently, there is no ``trusted certificate'').  You can accept this certificate permanently and will not be asked about it anymore.