Official releases of R source code are most likely what you want if you are looking for the sources of R (both Unix and Windows).

• The latest release: R-2.3.1.tar.gz
  Changes to the previous version are documented in the file NEWS (also contained in the sources).
• Older releases are available here.

Snapshots

Snapshots of R sources for development and patched release versions can be obtained as gzipped and bzipped tar files

• The latest development release: R-devel.tar.gz, R-devel.tar.bz2 (read what's new in the latest devel version).
• The latest patched release: R-patched.tar.gz, R-patched.tar.bz2 (read what's new in the latest patched version - look under patched!).

After downloading the R sources you should also download the recommended packages by entering the R source tree and running

    tools/rsync-recommended

from a shell command line.

The above archives are created automatically from the Subversion tree, hence might not even compile on your platform and can contain any number of bugs. They will probably work, but maybe not. Use them to verify whether a bug you're tracking has been fixed or a new feature you always wanted has already been implemented.

CRAN itself does no longer mirror source snapshots, because it makes only limited sense to propagate archives that change on a daily basis through a network of mirror sites. When a snapshot reaches one of the final nodes in the mirroring process, there are already 2 newer versions available at the orginal sites.

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