I've been wondering for years why people don't collaborate on lessons in the same open, ad hoc way they write Wikipedia and build open source software. Turns out I may have been using the wrong analogy. Teachers Pay Teachers is "... the first and largest open marketplace where teachers share, sell, and buy original educational resources", i.e., it's Etsy for teaching rather than Wikipedia. Their site says they have earned $200 million for their teacher-authors since their founding, and have two million "resources" available, ranging from lesson plans and interactive notebooks to games and classroom decor.

Thanks to Sumana Harihareswara for the pointer.