Exactly a month ago I read an article on Rails Spikes titled “Why programmers should play Go”. If you found that article interesting, and own an iPhone or iPod Touch with version 2.0 of the firmware, you will love Stones.
Right now Stones is for two players only, which means there is no AI, but it is still very fun. Plus, like chess, the best way to master Go is to play yourself.
How to play Go Learning Go Go on Wikipedia