Sue and Bruce Wilcox have written books and software to teach and play the game of Go.
I have found all of their material extremely useful, easy to use, and mentally challenging:
EZ-GO Oriental Strategy in a Nutshell – This is truly an excellent book and by far one of the best I’ve seen on how to play the game.
Bruce Wilcox’s Go Dojos: Sector Fights and Contact Fights – These are programs which walk you through in a tutorial like fashion of sector fights and contact fights in Go. Extremely well done. Hours and hours of explanations and exercises.
EGO Go program – A computer Go program that plays at about 12kyu. No, I’ve not beaten the computer yet!