What sites can I use to access free repositories of collaboratively edited text-books, curriculum, class notes and courseware?
Some good ones that I'm aware of:
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I usually stick to Wikipedia and occasionally WikiBooks for most of my general reading. From there, I'll usually jump around to the different sources cited for more in-depth material.
For professional topics, I turn to the IEEE and ACM, since I'm a member and have access to their libraries and e-courses. Depending on your field of expertise, I would recommend joining a professional society if one exists - their digital libraries alone are usually well worth the cost of membership and the content is professionally produced by the leaders in the field. These aren't usually collaboratively edited, but they are sometimes textbooks, white papers, and course material (depending on the specific organization).