America: The Story of Us is a fabulous documentary series from The History Channel. Each episode covers a significant event or period from U.S. history. At this time (August, 2011) it is available on Netflix streaming. A great supplement to history texts!
Mythbusters: What a fantastic way for kids to see that science is fun! The Mythbusters cover a huge variety of science topics on their show and stay amusing throughout. While their scientific method isn’t always perfect, these guys do a great job of showing the creative processes involved in science experiments.
National Geographic Documentaries: Most people are well aware of how abundant and awesome National Geographic documentaries are. What you might not know is where to find them. On this portion of NatGeo’s website, you can view many full episodes from various series (including Naked Science and Known Universe). There are also tons (currently 76) of various documentaries from National Geographic available at SnagFilms. Another option is Netflix….they have an ever-expanding selection of NatGeo specials and series.