What a great set of videos from a trusted online resource - PBS Learning Media. They have launched what they have called "Crash Course" for various topics for grades K-12. PBS Crash Course Site. The PBS site is more limited than their YouTube Channel, so check out both.
Here's a screenshot of the various topics:
Each topic includes a series of short videos (about 10 minutes each) on major subtopics of interest. The videos are engaging and humorous at times in order to help keep student interest. Check out their list and see what you think. The selection is limited to the topics show right now, but they may add more as time goes on.