GNU Go is a free software program by the Free Software Foundation that plays Go. Its source code is quite portable, and can be easily compiled for Linux, as well as other Unix-like systems, Microsoft Windows and Mac OS X; ports exist for other platforms. The program plays Go against the user, at about 5 to 7 kyu strength on the 9x9 board. Multiple board sizes are supported, from 5×5 to 19×19.