Rybka 4

The interface isn't brilliant and has much room for improvement. It's ok once you get used to it but could be much better and more intuitive.

The chess engine on the other hand is phenomenal. Seems literally invincible when used to cheat at internet chess. I tested it on freechess.org against another computer with a ranking of about 2250 and it won in under 20 moves.

In fact I reckon if you use this for internet chess cheating then the only players who will ever beat you is someone else using Rybka 4 who owns more powerful computer.

I've stopped using it because I got bored of just winning every single game.

The version I have is just standard Rybka 4. Deep Rybka 4 is more expensive but the engine makes use of multi-core processing so will be even better.