CrashCourse/SciShow - Two AWESOME educational channels, the former by John Green (of The Fault in Our Stars fame); and the latter by his brother Hank. Both channels have expanded from their initial narrow focuses to feature accessible content of varying lengths about a wide variety of topics.
Ted-Ed - TED-Ed takes the concept of the TED talk and brings it into the classroom. Most of their videos are in the form of short, five-minute animations covering a plethora of topics ranging from language to history.
TestTube News - TestTube probes deep into topics making the news and provides explanatory and challenging commentaries in short, digestible videos.