Free 9*9 version of David Fotland's Many Faces of Go. Strongly recommended as the best free introduction to Go for those who have Windows. Includes an introductory tutorial about Go. Supersedes Igo, which was a Dos program.
Free.
Runs on Windows.
Described (by its creator) at http://www.smart-games.com/igowin.html.
Can be downloaded from http://www.smart-games.com/igowin.exe.