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
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.
- RSXNT & RSXNTDJ
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.
Jose Fonseca's Gnu/Unix utilities for Win32
GNU tools documentation in winhelp format, ported non-standard
development utilities, Win32 debugging facility samples, etc.
See links to 'native' tools here.