Large collection of GNU and Open Source tools which provide functionality similar to a Linux distribution on Windows.
Cygwin is a a large collection of GNU and Open Source tools which provide functionality similar to a Linux distribution on Windows. A DLL (cygwin1.dll) which provides substantial POSIX API functionality. The Cygwin DLL currently works with all recent, commercially released x86 32 bit and 64 bit versions of Windows, starting with Windows Vista.
It is a Unix-like environment and command-line interface for Microsoft Windows. Cygwin provides native integration of Windows-based applications, data, and other system resources with applications, software tools, and data of the Unix-like environment. Thus it is possible to launch Windows applications from the Cygwin environment, as well as to use Cygwin tools and applications within the Windows operating context. WikiPedia