Gambit is a set of software tools for doing computation on finite, noncooperative games. These comprise a graphical interface for interactively building and analyzing general games in extensive or strategy form; a number of command-line tools for computing Nash equilibria and other solution concepts in games; and, a set of file formats for storing and communicating games to external tools.
http://www.gambit-project.org/doc/intro.html#section-downloading