Freshmeat page: http://freshmeat.net/projects/cygwin/
Cygwin is a DLL which provides a Unix emulation environment for Windows. The Cygwin environment provides a complete port of such development utilities as gcc, binutils, gdb, make, etc., as well as a number of useful utilities.
Besides making it easy to port Unix utilities to Windows, it provides a Free compiler which can build traditional Win32 applications as well.
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