I was always interested in understanding how Google managed to do its magic... If you visit Google Technology, they would have you believe that their patented PigeonRank technology makes it all happen. :)
But John Lam over at his blog IUnknown has just resurrected an April 2004 post from skrenta which contains an outstanding description of the Google architeture along with some great discussion (via comments). John also points to an article documenting the Google File System which makes an interesting read.