UWIN (Unix for Windows) is a collection of softwares created by AT&T Laboratories can be seen as an alternative to Cygwin. UWIN provides a unix-like environment for windows, complete with unix-emulation api and Korn shell.
UWIN is generally useful for developers who wants to compile certain unix applications in windows. Much like Cygwin, UWIN contains a DLL file POSIX.DLL that provides substantial unix API functionality instead of windows built-in posix layer. UWIN package also include files and unix development tools such as C compiler (but not gcc, obviously!).
The unix-environment also comes with Korn shell (AT&T ksh) and various other standard unix utilities which brings you the right unix-like environment in your Windows system.
You can download UWIN from AT&T AST/UWIN open source downloads.. UWIN is licensed under Common Public License 1.0.
p/s: this may be a great environment for those who wants to try out Unix environment without GNU stuff (even without gcc!).