Amazon fascinates me. The company manages to build services around what most companies would have historically just considered cost centers, and the Amazon Web Services operation is no exception.
AWS comes out of Amazon's own need to build out its infrastructure; they just figured out how to monetize it. In the process, they've managed to pretty much own the cloud services world, playing host to some of the world's biggest sites.
I recently found a fascinating (if hype-filled) listing of many of the sites Amazon hosts and what makes it even more interesting is there are videos from many of the IT managers of those sites explaining how they've used AWS.