Related projects

DJGPP is LGPLed POSIX/Unix runtime library and development system for MS DOS.
CygWin is GPLed POSIX/Unix implementation for Win32, with quite comprehensive coverage, fame, and support. Not very efficient, especially on Win9x.
UWIN is free for non-commercial usage, but proprietary (closed-source) implementation of POSIX/Unix API for Win32, with good coverage, but undercomprehensive toolchain.
Mingw is Minimalist Gnu-win32, public-domain system to develop native applications which use subset of POSIX/ANSI C provided by OS vendor. It aims for more POSIX coverage, but compromising with native conventions/limitations.
Further POSIX extensions for Mingw, to allow it to be autoconf target. Other extension is emulatation of native compiler. Generally, developed further than PW32 (for example, it has sockets and termios support). However, it's still based on vendor's libc, which means some inherent problems which will be needed to workaround. Also, author declines that his aim is to provide full-fledged POSIX implementation.
Toolchain to compile Win32 programs with EMX (of OS/2 fame) (RSXNT) or DJGPP (RSXNTDJ). Contains some utilities not available in any other package. Also, last versions undergone licencing changes and available under GPL/LGPL.
Project to elaborate POSIX environment for OS/2, basing on EMX library and BSD software/licencing.
Cross-ELF is a standalone library for loading ELF objects.
You guessed! LINE Is Not an Emulator. Da tool to run Linux apps on 'doze. Uses Cygwin.

Related sites

Jose Fonseca's Gnu/Unix utilities for Win32
GNU tools documentation in winhelp format, ported non-standard development utilities, Win32 debugging facility samples, etc.

Related tools

See links to 'native' tools here.