An executable installer for the Rtools collection, the MinGW-64 tools, and the "extras" (TCL/TK, bitmap code, internationalization libraries) necessary to build R in Windows is available for download here. Comments are welcome, to "murdoch at stats.uwo.ca".
This installer DOES NOT do a complete setup. The Rtools.txt file describes the remaining optional packages.
Links to various versions of the installer are on the main page.
Source code for current versions of the tools is available in the sources-tools directory. Source for previous versions is archived and available on request.