I guess you have never heard about Takat, but I am sure, that if you like logical board games, you are going to like it. 

Features:

• play with human or with 8 - level real AI computer!
• save state of your game in every moment!
• choose small or large board - every size involves diffrent strategies!
• use HINT button if you need help!
  
• be smarter than computer!
• have a much fun beating CPU while playing white and black stones!
• enjoy it as much as you want - it is freeware!

[UPDATE 03/15/2008] - new version 0.9.1 released! Go to the Download section and get it now!