Apple's free streaming video is pretty much all educational - but I've lost countless hours watching Stanford's series on Energy - in particular, I highly recommend Their Global Climate and Energy Project. Start with Solar Energy 101 - and see if you can avoid being hooked.

Obviously - there is also all of the podcasts from NPR, 5by5.tv, etc...

I've been happy with the free content I get from Apple - I probably spend, on average, 6-8 hours a week listening/watching it.