The Najdorf Variation is the most popular variation in the Sicilian Defense and is one of the most analyzed openings in all of chess. Black looks to thwart many ideas from white by playing an early a6. This passive looking move is actually quite aggressive as black has ideas to attack relentless on the queen side by stopping white from attacking.
more material at http://www.thechesswebsite.com
Rybka/Aquarium (software) can be found at http://www.chesscentral.com and http://www.chessok.com