Discovery.com
Unfortunately very few shows offer full episodes, but some (How It's Made) offer so many clips they might as well be full episodes.
It should work with most of the discovery.com sites. I've tested all the stuff that I'm interested in, but let me know if you are having problems trying to use it.
Add it as a web resource.
Example URLs:
http://dsc.discovery.com/tv-shows/mythbusters
http://science.discovery.com/tv-shows/how-its-made
http://animal.discovery.com/tv-shows/my-cat-from-hell
Updated 12-Aug-2013 - Discovery modified their episode list to include different quality streams
Updated 22-Feb-2014 - EPISODE_LIST location changed from science.discovery.com to http://www.sciencechannel.com for some episodes
Updated 23-Feb-2014 - Removed playlsit from the the episode list filter and handle empty video list