http://www.researchgate.net/ ResearchGate is a network dedicated to science and research.
http://www.studymode.com/ StudyMode offers book notes, free essays, course notes, flashcards and other tools.
http://www.slideshare.net/ Offers users the ability to upload and share publicly or privately Power Point presentations
http://www.scoop.it/ Discover incredible content through smart searches and the Scoop.it community.
https://delicious.com/ A social bookmarks manager. Using bookmarklets, you can add bookmarks to your list and categorize them.
http://www.teachertube.com/ A free community for sharing instructional videos and content for teachers and students.
https://www.flickr.com/ Flickr is almost certainly the best online photo management tool.
http://doaj.org/ DOAJ is an online directory that indexes and provides access to quality open access, peer-reviewed journals.
http://www.doabooks.org/ Directory of Open access Books is provided by OAPEN Foundation
http://textbookrevolution.org/index.php/Main_Page Free textbooks on the net
http://eric.ed.gov/ Free teaching materials
http://med-ed-online.net/index.php/meo Medical and Basic Science materials