Amazon Web Services has just named the seven finalists for this year’s $100,000 Startup Challenge. The prize will go to the most innovative startup built on Amazon’s cloud-computing infrastructure. The winner will get $50,000 in cash, $50,000 in AWS credits, and maybe even an investment from Amazon. The finalists are:

Encoding.com—Video encoding over the cloud.

Knewton—Online test prep service for standardized tests like the GMAT. Its adaptive testing/adaptive learning engine can be applied to any educational publisher’s content.

MedCommons—”cloud-based Health 2.0 application services for patients and doctors.”

Sonian—Archives and indexes emails that companies need to keep for compliance reasons, making them easy to search later.

Pixily—Document management for both scanned and electronic docuiments.

Yieldex—Online ad optimization engine for Web publishers.

Zephyr —online tool for managing software quality testing

Which one do you think should win? Just based on traffic, it looks like Encoding, Knewton, and Pixily are the strongest contenders. (Last year’s winner was Ooyala, which edged out Justin.TV).