Tarballs for the mingw-w64 sources are hosted on SourceForge. The latest version from the 4.x series is 4.0.6. The latest version from the 3.x series is 3.3.0. Winpthreads has been merged into the main tarball as of 3.1.0.
Details on how to get the mingw-w64 code from Git and an Git-web viewer are available on SourceForge.
The existing Darwin binaries have been built through buildbot in 2013 and links to them can be found on the dedicated page.
The OpenSUSE Linux distribution also has a large and well-maintained set of packages for cross-compilation.
A very large number of other binaries and sources are hosted in the File Release System on Sourceforge which might have what you are after.