Extrapolating
Our new Introduction to Statistics in Healthcare Research course has been massively enhanced by the inclusion of XKCD cartoons, stats courses require a bit of light relief. All set to negotiate the copyright clearance when we discovered they are cc licensed. I love Creative Commons. All I can say is anyone who is involved in developing a stats course should be using these.
Work found at http://xkcd.com/605/ / CC BY-NC 2.5