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: Still more metafile support in GdiPlus. Indirect draws support in Direct 3D. Calling convention on ARM. Improved serial port detection on Linux. Services on WoW64. Better DPI scaling in the Shell Explorer. Various.