Win32

Cygwin
Linux-like environment for Windows making it possible to port software running on POSIX systems (such as Linux, BSD, and Unix systems) to Windows. News, documentation and software downloads.
http://www.cygwin.com/

DJGPP
DOS port of the GNU C/C++ compiler and tools; 32 bit, highly optimizing, up-to-date, and free (GPLed).
http://www.delorie.com/djgpp/

GnuWin32
The project provides Win32-versions, native ports, of GNU tools or tools with a similar open source licence. Does not include compilers. Docs, downloads and links to related sites.
http://gnuwin32.sourceforge.net/

Hamilton C Shell
Recreates original Unix C shell and utilities fully from scratch, adds many enhancements, for Windows 95/98/Me/NT/2000/XP, OS/2.
http://www.hamiltonlabs.com/cshell.htm

Interix
Provides cross-platform interoperability applications, development kits, solutions and tools.
http://www.interix.com/

MinGW: Minimalist GNU for Windows
Compiler system uses GCC to produce Windows programs. Win32 ports of GCC, GDB, binutils to build native Win32 programs that rely on no 3rd party DLLs. Website also contains MSYS, a Minimal SYStem, a shell, with which a configure script could be executed.
http://www.mingw.org/

MKS Unix-Windows Co-Existence Products
Toolkit suite of Unix and NT utilities for Windows, X Windows display environment, NutCracker development, interoperability, runtime environment.
http://www.datafocus.com/

Porting Unix to Windows NT
By David G. Korn;, 1997, USENIX paper (abstract, full text requires membership) describes AT&T UWIN project.
http://www.usenix.org/publications//library/proceedings/ana97/korn.html

PW32 the POSIX-over-Win32 layer
Implementation and subset of POSIX/Unix API for Win32 systems. Focus: efficiency, full platform coverage, with good support for low-end Win9x systems. Based on DJGPP runtime library by DJ Delorie. [Open Source, LGPL]
http://pw32.sourceforge.net/

UnxUtils
Native Win32 ports of some GNU utilities. Does not include compilers.
http://unxutils.sourceforge.net/