Perdue's "Online Writing Lab" gives background and information on many different writing techniques and grammar material.
https://owl.english.purdue.edu/owl/
Indiana University of Pennsylvania has a helpful set of writing links on their website. Their links provide guidelines in many areas of writing, such as MLA format and grammar guides.
http://www.iup.edu/page.aspx?id=62333
The Oatmeal is a humorous blog that has a series of helpful comments that explain grammar rules and techniques in a fun and easy to understand manner.
http://theoatmeal.com/tag/grammar