Unix for WINdows:U/WIN is software that lets Unix programs be recompiled and run on Win32 systems: Windows 95-98-Me/NT-2000-XP. Descriptions, documents, downloads.
DJGPP: DOS port of the GNU C/C++ compiler and tools; 32 bit, highly optimizing, up-to-date, and free (GPLed).
GNU Emacs for Windows NT and Windows 95/98:
GNU Software for MS-Windows and MS-DOS: Book and CD from the Free Software Foundation
GNU-Win32 Related Projects: Mumit Khan's comprehensive page on Mingw32, Cygwin, UWIN, and related tools and projects.
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.
Interix: Provides cross-platform interoperability applications, development kits, solutions and tools.
Mingw32 FAQ: Information about the Mingw32 compiler.
MKS Unix-Windows Co-Existence Products: Toolkit suite of Unix and NT utilities for Windows, X Windows display environment, NutCracker development, interoperability, runtime environment.
Porting Unix to Windows NT: By David G. Korn;, 1997, USENIX paper (abstract, full text requires membership) describes AT&T UWIN project.
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]
Windows NT Services for Unix: Microsoft product suite; has list of features, system requirements, evaluation tools, common questions, support, related books/products, pricing.