mingw-w64

GCC for Windows 64 & 32 bits

Sources

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.

The old 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.

Unsorted complementary list

Darwin/Mac OS X

The existing Darwin binaries have been built through buildbot in 2013 and links to them can be found on the dedicated page.

OpenSUSE

The OpenSUSE Linux distribution also has a large and well-maintained set of packages for cross-compilation.

Store of binaries on SourceForge

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.