Here is the link to Harvard College Writing Center: http://writingcenter.fas.harvard.edu/.
It is a very helpful website for academic writing, including comprehensive information and resources about academic writing, from essay structure, to strategies and principles of writing different parts of an essay. It also has guides about how to write in different disciplines like psychology, sociology, etc.
The Purdue Online writing lab: https://owl.english.purdue.edu/owl/section/1/.
It is especially helpful for those who need to write papers and need to comply with APA form. It also includes APA sample paper.
A great resource for K-12 writers: http://www.readwritethink.org/classroom-resources/student-interactives/.
From this website, students can find interactive tools to help them organize their thoughts when writing. Tools on this website include Story Map, Persuasion Map, Word Family Sort, etc.