Zediva, the newest contender, does things a little bit differently by streaming actual DVDs.
When you stream a movie from Netflix, Amazon, Vudu or any of the other streaming services available, you're watching a file that's stored on a massive server. When you stream a movie from Zediva, you're watching an actual DVD that's in an actual DVD player.
The latest Netflix competitor boasts that by using actual DVDs instead of digital media files they're able to do a few things that others can't. For starters, Zediva gives you access to movies on the day of release instead of making you wait for the 28 day window.
Even better, Zediva is making special features available. At the moment, you can only access subtitles and audio tracks, but that does give you access to commentary on DVDs that offer it. Other features will be made available in the future.
HD movies aren't yet available, thought Zediva does plan to expand to Blu-ray.