I just mentioned Amazon EC2 the other day, as being in limited beta--now Amazon's Elastic Compute Cloud (Amazon EC2) is open to the masses in an unlimited beta. EC2 "is a web service that provides resizable compute capacity in the cloud. It is designed to make web-scale computing easier for developers."

From the EC2 page:

Amazon EC2 presents a true virtual computing environment, allowing you to use web service interfaces to requisition machines for use, load them with your custom application environment, manage your network's access permissions, and run your image using as many or few systems as you desire.

So now you can create your own virtualized data center, optimized for the web, and it'll cost you next to nothing, at least to start. Amazon's AWS calculator lets you estimate your usage of the Amazon Web Services, which include EC2, Amazon Simple Storage Service (Amazon S3) (that's middleware, and worthy of a separate discussion here, soon) and Amazon Simple Queue Service (Amazon SQS). If you think you know what you need, you can figure out about how much it'll cost you, monthly.

You get the use of Amazon's infrastructure, worth who knows how many millions of dollars, but you pay for what you use. Kind of like having a limo/private aviation service available on-demand, where you pay only for what you use, when you use it--but you get instant transport in state-of-the-art cars/planes/helicopters.