MIT + K12 - A collaboration with Khan Academy (another great pick) to bring high quality content to the K-12 classroom.
TED-Ed - Of course, the TED Talks channel is a winner but TED-Ed does a great job with animation and videos for learning.
NatGeo - National Geographic has wonderful content in many categories and the videos are always high quality.
PBS - How do you go wrong here? Check out PBS Kids for little ones, too.
YouTube Teachers - Want to learn more and connect with other educators passionate about blended learning, media rich classrooms, and engaging kids? Help to build the world's largest classroom here.
What did I miss? Please leave your favorites in the comments!