The web is full of top-quality documentary films that you can watch online for free. Trouble is, it's difficult and time-consuming to find, grade and categorize them, so you often end up watching something that's not up to scratch.
Which is why www.documentarystorm.com is so handy. The guys behind this site have done all the hard work of locating, quality-checking and categorising everything and, as you can see from the partial list below in the screen shot, there are loads of topics to choose from. In total there's more than 2000 videos online, all available to watch in your web browser.
The site is free to use, too.
My thanks to Will for suggesting this site, in a comment to a story about a similar site that I published earlier this year. That site, at http://watchdocumentary.org/ is also still going strong too.