GCC for Windows 64 & 32 bits
This is an old revision of the document!
Tarballs for the mingw-w64 sources are hosted on SourceForge. The latest version is 3.3.0. Winpthreads is available in the main tarball as of 3.1.0.
The wiki has instructions for building native and cross toolchains.
Details on how to get the mingw-w64 code from Git and an Git-web viewer are available on SourceForge.