This is my favorite version on the Sicilian and has demonstrated the best chance for black to equalize whites statistical advantage. In fact in today's game d4 is played so often to just avoid playing the e4-c5 line. The intention with 5.a6 is to precede e5 on the next move to put a pawn in the center. By playing 5. e5 before 5.a6 you can be met with 6.Bb5+, here black must either play 6.Bd7 or 6.Nbd7. 6.Bd7 allows white to take blacks bishop, now the d5 square becomes weak. With 6.Nbd7 white can play 7.Nf5, now black can only save his d pawn by playing 7.a6 8.Bxd7+-Qxd7. In both of these scenarios, whites position is better. By playing the Najdorf 5.a6, black takes away the check on b5 and in this way e5 is possible for his next move. Also by moving 5.a6 this keeps both of whites knights from establishing a post on the b5-square and also black creates some play on the queenside by looking to move b5. Whites most common response in answer 5.a6 is 6.Be3 looking for moves f3, Qd2, g4 and 0-0-0 in some order. Black can respond with 6.e6, 6.e5 or 6.Ng4. White can also play 6.Be2 to enable the kingside castle, or 6.Bg5-e6 7.f4, here white looks to attack the knight with 8.e5. Black can break the pin on the knight meeting 7. f4 with 7.Be7, or he could play 7.Nbd6. 7.Qb6 is fine as well. By moving 7.b5, instead of the before-mentioned moves, white has the idea to next move 8.e5-dxe5 9.fxe5-Qc7 10.exf6-Qe5+ winning a bishop for a knight. Some other variations for white are 6.g3, 6.h3, 6.f4, or even 6.Bc4 which Fischer used often in his games. I hope I wrote that correctly and in a fashion easy to grasp. A member asked me to put up a lecture of sorts. If you would like to see more of these or any answers to a specific opening or position please post it. It would generate nice discussion.

 
Here is a game played where I lost as white against the Najdorf, showing its ultimate effectiveness. In this game I played 6. Bg5 then 7. f4. But ulitimately I was too passive and missed the key 8. e5 move. This variation was the main line of the Najdorf until recently. In most games I play 6.Be3 instead of 6.Bg5 with better results.