Free As In Pretty Much Whatever You Want
A couple of different people have asked us recently whether they can use our materials in their courses. The answer is an emphatic "yes": all of our slides, essays, posts, and what-not are covered by the Creative Commons — Attribution license, and all of our code is covered by the open source MIT License, so basically, you can do whatever you want, whenever you want, without seeking special permission. (And yes, this means you can charge for your courses, even though we don't charge for ours.) The only requirement (imposed by the CC-BY license) is that you cite us as the source, and of course we'd be grateful if you'd point us at your material and/or send us any changes you make.