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:
      - General name: r-project.org
      - Serial number: 04:9D:4F:0D:53:03:DB
      - Valid: from  03/10/2014  to  03/10/2016
      - Issuer: Starfield Secure Certificate Authority - G2
                http://certs.starfieldtech.com/repository/
      - SHA1-Fingerprint: F5:15:96:DB:F1:2F:35:1B:96:06:C3:A4:2A:E2:80:78:76:4C:A4:30
      - MD5-Fingerprint: D8:7D:4F:8F:ED:92:65:EE:EE:A5:73:2D:BB:F6:35:E6