Diaballik is a game I've been playing and have yet to beat it. It's a strategy game that looks a big rugbyish in nature... actually it reminds me of a board game I have called Street Soccer.
It's simple really, but the AI is quite good and has had me on the ropes since starting to play this game. The game offers two modes of play (minor rules difference). My only concern is that perhaps once I do win I'll have figured out how to beat the AI and that will be that, but that's speculation. You do always have the option of playing against another human in hot seat mode. Games that combine sports with strategy are difficult to do well. There's nothing fancy about the graphics or sound here... it's just good, strategic game play. If you like games that require thought instead of reflexes give this a shot. And did I mention it's free?