Version 1.0 of libffi-d has been released. This library is a wrapper around the C library libffi which is useful for dynamic invocation of native functions. Since libffi-d provides a more 'D-like' wrapper around the C library, as opposed to just being a binding, there is a certain amount of logic in libffi-d that is actually quite error-prone. It has been battle-tested in MCI's interpreter on Windows, Linux, and OS X.
Archives can be downloaded here.