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: Better support for grayscale mode in DirectWrite. Per-application StartupWMClass in desktop files. Virtual memory compatibility improvements. Palette handling improvements in WindowsCodecs. Reply messages improvements in WebServices. Various.