Rather than rewrite all of the Gnu tools to use Win32 system calls, the Cygwin developers wrote a POSIX compatibility layer and encapsulated it as a DLL. The DLL includes the most common *nix system calls. Unix code built under Cygwin links against the DLL. A lot of *nix code builds "out of the box" under Cygwin.
The Cygwin distribution includes the GCC compiler suite, plus all of the standard development tools, including things like ls, cp, and mv, the bash shell, and the Vim, Emacs, and Xemacs editors.
A full Cygwin installation provides a complete Linux compatible development environment under Windows.
Homepage: http://www.cygwin.com
License: GPL
Availability: Win95, Win98x, WinNT?, Win2K, WinXP? (2K/XP recommended)