A recent issue of Information Week carried a long piece on 'inside Google' describing some of the technologies they use. Most of it is open source, such as Linux and 'commodity servers'. The article goes on to describe how they 'Googleize' the software and hardware to maximize performance. 

The item that caught my eye was the competetive advantage of using open source software to mask the size of your operation. Since they don't buy operating systems, etc. no one can tally the numbers to get an estimation of how big their operation is.

The whole article is available in PDF format here