Crash Course is a YouTube channel that was created by Hank and John Green.  Hank Green explains Chemistry, Ecology, and Biology.  John Green explains US History, World History, and Literature.  I really like the series of videos because they are only around 10 minutes long and the lesson gets taught in a creative, visual way that engages students.

The video embedded below is the latest lesson uploaded to the Crash Course YouTube Channel.  Hank explains Unit Conversion & Significant Figures: Crash Course Chemistry #2