I've used Dora the Explorer and Go Diego Go for spanish lessons, and varied documentaries for different things. TedTalks for Kids is fun for "filler" when I want something quick but educational (they are about 15-20 min ea at the most) Also, on occasion, if there is a movie based on a book we have read then I will have them watch the movie and point out the differences/similarities. We stream Netflix through our wii, which also has a YouTube channel now, so I will occasionally use YouTube instead. It depends on the subject we are working on.
We watched The entire Walking With Dinosaurs series, Zave watched the documentary The Dark Ages, and we're looking into the food documentaries for our Health and Nutrition units later. DH and I make sure that we pre-view everything because some stuff is inappropriate for our age groups (4, 6, and almost 8)
Mythbusters my ds's favorite right now, but he also enjoys military docs and anything survival related. We've used a lot of PBS docs and some of the History Channel ones. Its easier now since my kids are older, I'm not having to filter as much material now. There is a lot more to choose from now. Its hard to find science-themed ones that teach from a Creationist standpoint, so we just kind of avoid the science ones.