As an open-source project, the R source code has always been available to download from the R-project website. You can find source code for the latest released version here, and for the changing-daily new version in progress (R-devel) here.

But if you don't have the R sources handy, and just want to check on the contents of a file or two, a read-only, browsable mirror of the R-devel sources is now available at GitHub. This mirror is updated hourly from the SubVersion repository that the R core team uses to manage the R source code, and so also includes the change history and commit logs for each file. (You'll see a lot of notes marked [ripley] and [hornik] as they and other R-core members update the code.)

Note that this is the source code for the development version of R, and so probably doesn't match the version you're using. But it's a great reference to see what's going on in the R sources right now. Also, it's only includes the files used for development; some human readable files which are built from other source files aren't included. (One example is the HTML version of the NEWS file, but you can always find the daily NEWS file for R-devel here.)

github: Read-only mirror of R source code (via Hadley Wickham)