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: Async interfaces and ACF files in the IDL compiler. Support for substorage transforms in MSI. RPC/COM marshalling. Support for Unicode requests in WinHTTP. Shell Autocomplete optimizations. Various.