SnagFilms is a free, universal iOS app that brings hundreds of independent films to your iPhone, iPad or iPod touch. You search by category or title. SnagFilms offers a nice collection, and I even found a pair of documentaries about Apple.
The app is ad-supported, so you'll see an ad before your movie begins and at least another during the presentation. I understand the need for advertisements, but SnagFilms presented the same ad repeatedly, which detracts from the experience.
I use SnagFilms to watch Macheads and a documentary about the race to the moon on my iPad. The quality of audio and video was fine. The developers say they are adding new films all the time.