Smithsonian Channel is available on Roku devices as a free channel with some of their documentaries.
Not sure if it will always be free since it is a joint venture between the Smithsonian and Showtime.
(some of the Smithsonian shows recently began showing up on netflix too).
The shows are documentaries. Reminds me of the History channel's documentaries before the history channel began loading up on all of those reality shows.
Smithsonian Channel: Telling America's Stories