For academic writing purposes:

http://turnitin.com/ (Free sign-in through your instructor’s class list)
http://www.buowl.boun.edu.tr/
http://owl.english.purdue.edu/
http://www.essex.ac.uk/myskills/Plagiarism_and_how_to_avoid_it.pdf

For good presentation skills:

http://www.essex.ac.uk/myskills/skills/presentations/default.asp
http://www.canberra.edu.au/studyskills/learning/oral#dealing

Some classics:

http://www.gutenberg.org/wiki/Harvard_Classics_(Bookshelf)

Online courses:

https://www.edx.org/
https://www.futurelearn.com