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These are just on my iPod. On the AppleTV I subscribe to dozens more. Mostly the HD shows, but there's tons to choose from. NASA JPL, PBS, Discovery, Revision 3, and hundreds of independent shows. Everything from learning guitar, to cooking to world history.
The lastest AppleTV firmware makes it nicer because it will finally play podcasts and movies intended for iPod viewing (240 x 320). The original firmware would balk and say "incompatible format". It only took them two years to update it, or something like that. Anyhow, podcasts are fantastic. And when you think about it, they lower the entry point to make it possible for almost anyone to produce their own shows. Viewers can decide what they like instead of some faceless suit in a penthouse suite doing it for us. We're still not a true Democracy, but every little bit helps get us a little closer.