Cloud Computing. Available at Amazon.com Today

Amazon is notoriously tight with operational data. But the company does provide a few figures, and one shines a light on how far Amazon has come from the old Wal-Mart-in-the-sky concept. Almost a third of the store's total number of sales last year were made by people selling their stuff through the Amazon machine. The company calls them seller-customers, and there are 1.3 million of them. They're just one wave of outsiders to have washed over Bezos' threshold. The next is already breaking: Web developers itching to make their mark on the Net.

The transformation from storefront to platform happened by evolution rather than design. It began way back in the dialup dark ages, when Bezos and his crew were looking for a cheap and easy way to expand the store's reach. One solution was to let Web developers link directly to Amazon products on their own pages — and give them a 15 percent bounty on sales they generated. Today that program, Amazon Associates, is the Net's definitive distributed sales network, turbocharged by the world's biggest archive of product information, customer reviews, and recommendations. One spooky Associates service dubbed Omakase — Japanese for "Leave it up to us!" — offers up Amazon product ads keyed to your site's content, your audience's response to earlier ads, and (gulp) the specific viewer's past Amazon purchases. Google AdSense with a kickback.

Turning other people's sites into Amazon storefronts set a stage for the next leap: letting outsiders sell their own stuff through Amazon's platform. Third-party selling began in 1999 as a jittery response to eBay, which looked smart for avoiding handling things physical altogether. Amazon's auctions were a bust, but they kicked off what is now the thriving Amazon Marketplace ("Sell Your Stuff"). The company earns a cut of up to 15 percent from sellers who opt for plain-vanilla product listings ("12 used & new from $58.99"). High rollers can sign up for top-of-the-line Fulfillment By Amazon, which parks their goods on the shelves (45 cents per cubic foot per month), then slips them into Amazon's own shipping stream. If you're Target or the NBA (or, until recently, Borders), you can sit back and rent a world-class ecommerce operation, from shopping cart to warehouse to delivery.

The rationale for all this, in Jeff-speak, is "totally not abstract." Meaning it's a no-brainer. "If you're trying to have not just every book but every out-of-print book, every indie-band CD, and every vinyl record, there's no way you're going to do it on your own. You have to open up."

That turns out to be harder than it sounds. When Amazon plunged into auctions, its simple dotcom-era server-and-database combo was already staggering under the combined weight of growing complexity — recommendation engines, transaction tools, a steady rain of new features — and mushrooming traffic. Beset by problems with speed, reliability, and scalability, engineers first tried migrating the whole hairball to a mainframe. It didn't help. Then they gambled on a Net-centric idea just beginning to bubble through the computer science world: loosely joined software modules, each performing a specific function while exchanging information with the others. Bingo. Log in to Amazon's gateway today and more than 100 separate services leap into action, crunching data, comparing alternatives, and constructing a totally customized page (all in about 200 milliseconds).

The new architecture brought a huge improvement — along with an unexpected opportunity. "We were already building these really incredible, robust services for ourselves," says Andy Jassy, who wrote the original Amazon Web Services business plan four years ago and has been running the operation ever since. "It wasn't a huge leap to realize they could be valuable to other people." Thus Amazon's next new business: opening not just the store in the sky, but the very silicon and software that keeps it aloft.

Just across Highway 101 from the Googleplex in Mountain View, California, a banner fluttering from a nondescript building heralds yet another high tech dream: Ooyala. Inside, a dozen software engineers wearing headphones pore over screenloads of code while grazing on M&Ms and organic treats served in plastic tubs. The three founders, all Google emigres, are chasing a suitably big idea — interactive "hypervideo" with seamlessly embedded links. What they're not doing is blowing capital on shiny hardware. They built their television killer using little more than the desktops in front of them, plus major slices of computing power courtesy of Mr. Bezos. "You can't execute fast when you're running to the data center every night to fix machines," says Ooyala engineering chief Sean Knapp. "Infrastructure is the big guys' most powerful asset. This levels the field."