Rybka is a very strong chess engine. It is not available on chess.com. I personally would recommend Houdini 1.5 over Rybka, as Houdini is free and is just as strong as Rybka while getting Deep Rybka might put you back 150 dollars or so. Chess engines are very useful for analyzing your games, so it can be helpful to have a very strong one at your disposal.
What is rybka? I'm always hearing members talking about it, but what is it, and, is it on chess.com? If it is, how can I access it?