"Open Educational Resources are teaching, learning and research materials in any medium – digital or otherwise – that reside in the public domain or have been released under an open license that permits no-cost access, use, adaptation and redistribution by others with no or limited restrictions" -- The William and Flora Hewlett Foundation
Open Educational Resources by NUS Libraries
The above guide covers the following:
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OER Geography Textbooks
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"Geovisualization tools, APIs and libraries that will let you create beautiful web maps".