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National Science Digital Library
http://nsdl.org/
"The National Science Digital Library (NSDL) was created by the National Science Foundation to provide organized access to high quality resources and tools that support innovations in teaching and learning at all levels of science, technology, engineering, and mathematics (STEM) education.."
OAIster
http://oaister.umdl.umich.edu
"OAIster is a project of the University of Michigan Digital Library Production Service. Our goal is to create a collection of freely available, previously difficult-to-access, academically-oriented digital resources that are easily searchable by anyone."