Amazon (AMZN) has rolled out its new Amazon Video on Demand video streaming service. After a quick test, we think it's the best competition iTunes has seen yet. Amazon's new video service picks up where its old one, Unbox, left off -- it now works on Macs, for example, and video starts streaming instantly. How does Amazon's streaming service compare with iTunes, Hulu, and Netflix streaming? Like Apple's iTunes, Amazon's service will offer rentals for $3 to $4 and movie purchases for $10 to $15. Most TV episodes seem to cost $2; season packs are also available. Unlike iTunes, Amazon offers NBC shows -- some of the same episodes you'll be able to watch at Hulu for free, but without the commercials. And its library of 40,000 movies and TV shows blows Netflix's sorry selection out of the water. Also nice, but not a game changer: It'll stream to some Sony TVs and TiVo devices, as well as Windows Media Center extender devices, like Microsoft's Xbox 360. The downside: Full downloads for offline viewing are still only available for Windows PCs and TiVos, not Macs or Apple TVs. And portable movies still won't play on any Apple gadgets like iPods or iPhones.