Wine is an implementation of the Windows API on top of X and Unix. It does not require Microsoft Windows, but can use native Windows DLLs if they are available. It provides both a development toolkit for porting Windows source code to Unix as well as a program loader, allowing many unmodified Windows programs to run on x86-based Unixes.
minor feature: WinSpool, GPhoto, and a few other modules converted to PE. Better support for inline functions in DbgHelp. Beginnings of a MSDASQL implementation. Various.