Bison is a free implementation of yacc.
Bison is available for most of the UNIX-like platforms and as the bison package from the Cygwin setup.
If GNU Bison isn’t already installed or available as a package for your platform, you can get it at: http://www.gnu.org/software/bison/bison.html.
After correct installation, typing at the bash command line prompt:
$ bison --version
should result in something like:
bison (GNU Bison) 2.3 Written by Robert Corbett and Richard Stallman. Copyright (C) 2006 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
However, the version string may vary.
A native Win32 yacc/bison package can be obtained from http://gnuwin32.sourceforge.net/. The installation should be straightforward.