The Roku is a wee box that hooks up to your internet and TV over which you can stream movies and TV shows. Until recently, the Roku only worked with Netflix (the streaming is free and unlimited with your Netflix acct) but the Roku added support for Amazon's Video On Demand service the other day, bringing Amazon's 40,000+ movie titles into the mix. I have friends that love this thing.
BTW, Amazon is getting good at closing the loop on this stuff. Like Apple (Apple TV / iTunes Store), they're not only selling the media but also the device.