Amazon today announced Amazon Video On Demand, a new video service that lets customers watch ad-free movies and television shows on Macs or PCs.

Previously, customers could only download titles and watch them on a PC (using the Amazon Unbox application) or on their TiVo box. Now customers can either watch instantly within their web browser or download using the Unbox application and watch anytime.

Customers can also rent or purchase Amazon Video On Demand movies and TV shows directly through Sony’s BRAVIA Internet Video Link device on compatible BRAVIA television sets (currently in beta).

A free preview of the first two minutes of each movie is available.

According to Amazon, Amazon Video On Demand is the only U.S. digital video service that gives customers the choice to stream or download premium movies and TV shows. 40,000 licensed movies and TV shows are available through the service.

Amazon’s move raises the stakes in Internet television, because it looks like Amazon is building a platform that it will make available to a variety of TV makers, rather than a closed system like Apple’s iTunes/Apple TV solution.