sketchplanations
Jonathan Hey has created a brilliant site called sketchplanations.com where he explains the world one sketch at a time.
His sketches are one-page explanations of concepts, and licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License. That means you are free to use images such as: Confirmation Bias, The Swiss Cheese Model, and The Learning Pit in your courses with attribution. He challenged himself to create one sketch a day for 365 days, so there are a lot to choose from!
For an example, see his sketchplanation of The Abilene Paradox below.
