Anything other than Rybka Aquarium is a waste of money.
Chessmaster 9000
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I tend to disagree. Chessmaster is a great investment and really taught me a lot.
Well CM9000 is $10 at walmart (last time i looked), so you could see how most people even with just a passing interest in chess would be tempted to buy it.
Rybka is probably the best, but as they say, only for "the serious chess player".
aatkins - I disagree. Chessmaster comes with many excellent chess tutorials and a wide range of virtual opponents. If you are looking for these things and you buy Rybka Aquarium, you will be sorely disappointed. 
[This may be the first time I used Rybka and disappointed in the same sentence.]
CM9 is very good. It even looks good. Its interface is easier to handle than that of CM10. The only advantage of CM10 is the extra tutorials from Josh Waitzkin, especially on the endgame. The CM10 interface, supposedly clearer and simpler, isn't.
The only problem with CM9 is that you need the CD in the drive once every 14 days. For CM10 Ubisoft released a patch that allows you to play without the CD.
And, yes, CM9 is now selling for very little. In the Philippines, the equivalent of US$10.
What does everybody think of this CD?
Just making conversation.....first time I have posted......
RedZeppelin
In the days of my youth, I was told what it means to be a man,
Now I've reached that age, I've tried to do all those things the best I can.
No matter how I try, I find my way into the same old jam.