There's so much educational material on the web, it's hard to know where to start. The pure abundance of information is well-documented, so I won't go into that here. I will say that this abundance has led to the need for (and, I think, the coming age of) the curator.
A good curator does one thing: takes all the information relevant to a subject, distills it to its essence, and presents that distillation in a concise, informative way. That's precisely what Hubski user organicAnt has done. In his own words:
After years of documentary addiction I decided to build a website that brought together the best ones I had watched. I've compiled and categorised 100+ movies, including DVD covers, descriptions, trailers and the actual movies. All free and legit for everyone to enjoy during a boring Sunday afternoon. There's something for everyone's taste.
There is, indeed, something for everyone. Movies range from "The Problem with Atheism" to "The Known Universe" to a TED talk by Clay Shirky. Documentaries are categorized into topics like media, society, and agriculture. You can even sort films by title, release year, run time, and a bit more.
Most of the videos (if not all) are simply embedded YouTube links, which makes the site pretty simple: just click "watch movie," and the video will start playing without your having to leave the page. If a trailer's available, you can play that, too.
Keep in mind that this site was put together by one developer, and quickly, so there may be some bugs. Even so, Lucid Tree is a pretty great resource for those looking for a good documentary.