This is probably not a popular opinion, it is now wholly unnecessary and was often abused but I would say jQuery's source code. It took a language previously co

This is probably not a popular opinion, it is now wholly unnecessary and was often abused but I would say jQuery's source code. It took a language previously considered as a toy with different implementions that were inconsistant and unwieldy and made something immensely powerful and consistent with it. It brought lisp style functional programming to many who had never seen it before. Within a matter of months everyone working on the web was using it.