UGA databases and Free Sites
This is a UGA Libraries database containing over 7,000 educational films and newsreels. You must be on the campus network, or use the GALILEO password or your UGA MyID, to view the films. There are several documentaries from the BBC and PBS' American Experience.
PBS Video allows some free documentaries on their site, but some will be limited to PBS Passport users. PBS Passport access is a perk of donation. Check the pages for Nature, Nova, American Masters, Independent Lens, POV, and other PBS programs to see what's free. Some PBS shows will also be licensed to special Amazon Prime channels, such as PBS Living.
Commercial sites - Commercial sites require a subscription, and/or advertising to view videos.
Hulu - Check site for current account plans and pricing.
Netflix - Check Netflix for their current account pricing. Commercial films and TV series on Netflix tend to come and go due to contract changes. However, many documentaries can be found on Netlix consistently. Here are some suggestions of documentaries on Netflix -- worth a search to see if they are still there: Huffington Post, Top 150 from NonFics (2015). Also, many blogs and websites will run articles on these changes. A web search for "what's coming to Netflix" or "what's new on Netflix" usually turns these types of articles.
Box Office MOJO's list of top grossing documentaries - Note: Includes concert movies as documentaries