The folks at MIT surprised everyone several years ago when they suggested the concept of "open-curriculum". Their next step was to lead the way by offering their best material, at no cost.
Today, that is almost the standard. The global community is working together to create applications to involve everyone, and a great deal of this is free (and / or "open-source").
See the Wikipedia articles for a better understanding of the concepts. Then, experience the applications introduced below and consider how they could be leveraged in your school or classroom.
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