Every time you Google "Krispy Kreme cheeseburger" or "how to relieve herpes" or "laser cats," it takes the processing power of up to 1,000 machines.
In a talk at WSDM 2009, Google Fellow Jeff Dean revealed some statistics about how Google's operations have grown from 1999 to 2009, and it's pretty mind-boggling. Queries have gone up by a factor of 1000, yet search lagtime has dropped about 1000ms to just 200ms—nearly instant.
The reason a single search can take up to 1,000 computers now—instead of say, a dozen—is that Google's whole search index is held entirely in memory. Every time I try to think about how large that index must be I get a headache. I'm pretty sure if I concentrated on it too hard, my head would pop. [Labnol via Maximum PC, Image of old-school Google server via Coding Horror]