The winner of Amazon.com's second annual startup challenge, highlighting promising companies using its Amazon Web Services:

Yieldex, a service that helps online publishers do "accurate forecasting of overlapping online advertising inventory and optimal campaign allocation." The company's engineering office is in Boulder, Colo., and its sales office is in New York.

The company is trying to help large publishers (sites getting 1 billion to 10 billion impressions per month, such as CNN or the New York Times) get more sales from premium inventory attracting $10 to $30 CPM using its "proprietary yield index."

It's a nice enterprise showcase for the capabilities of Amazon: A single customer may send 20 gigabytes of data nightly for analysis, yet Yieldex uses one part-time system engineer to manage its services running on AWS.

Chief Executive Tom Shields -- who previously co-founded NetGravity, a company sold to DoubeClick in 1999 for more than $500 million -- said the cloud services gave Yieldex flexibility and scale on tap.

"We had no idea when we started this thing what kind of capacity we were going to need,'' he said.

The winner was announced tonight at an event at Bell Harbor after a year of Amazon traveling all over the place, pitching its Web services to developers.

Seven finalists were flown to Seattle to present their companies to a group of venture capitalists and Amazon managers who made the choice.

The prize is $50,000 cash and $50,000 worth of Amazon services.

Other finalists were:

Encoding.com, which "has transformed video encoding from a traditional software model to a software as a service (SaaS) platform," using Amazon elastic computing services. Chief Executive Greg Heil said that while YouTube has figured out encoding, but it continues to be a huge infrastructure challenge for second and third tier video sites.

When serving clients, Encoding activates Amazon server instances to handle their needs.

(Maybe Amazon will buy the company or add its own encoding service to its new content delivery service?)

Knewton, an educational content service with a "self-optimizing 'Darwinian' engine" that evolves as new students join its network and increase its dataset.

MedCommons, which provides "cloud-based Health 2.0 application services for patients and doctors, and enables third parties to customize and extend the MedCommons Platform for their own needs." It's a Boston-area competitor to Microsoft's HealthVault and Google Health that uses Amazon to store personal health record repositories.

Sonian, a "cloud compute e-mail productivity service" that archives and indexes electronic communications, files and unstructured content. After eight months in business it has more than 50 customers -- companies with 300 to 6,000 employees -- and white-label deals with vendors reselling its service.

Pixily, "an interactive document management service that organizes paper and electronic materials online." It's aiming to help consumers reduce paper clutter and better manage information, and provide businesses "an affordable on-demand document management service." The company scans receipts and other records for customers, who can then search them with a Googlesque search window at its Web site. Customers can scan documents themselves or take pictures of materials and e-mail them to Pixily, or send batches of documents to the company in prepaid envelopes.

Zephyr, which offers management tools for enterprise test departments. It aims to lower costs and increase productivity "with real-time visibility into all aspects of their software quality cycle." Founder Samir Shah said customers such as Accenture, Infosys and Cap Gemini are using its Amazon-powered services to provide service to their customers. He's a fan of Amazon's pay-as-you go computing.

"I love being nickel and dimed here -- it's really great," he said.

Last year's winner was a video ad service company called Ooyala, which was started by ex-Google employees. A few months after the Amazon.com award, it raised $8.5 million in its second round of funding.