The Many Faces of Go

From the developers homepage:

"The Many Faces of Go" is the 2008 World computer go 19x19 and 9x9 champion, 2002 World Computer Go Champion, 1998 World Computer Go Champion, and ten time US Computer Go Champion. It is one of the strongest Computer Go opponents in the world, and has many other great features including an integrated IGS client so you can play on the Internet. A sophisticated set of go problems and a Joseki tutor help you improve your game. It includes several go players of different strengths, from 2 kyu to 18 kyu, so players of any level can have a challenging game.