Writer's Web (published by the University of Richmond) - This site is organized to reflect the stages in the writing process and links to topics such as getting started; writing first drafts; focusing and connecting ideas, analysis and argument; peer-editing strategies; sentence structure and mechanics; editing for clarity and style; and documentation and punctuation http://writing2.richmond.edu/writing/wweb.html
ESL Blues: The ESL Blues site offers overviews on a wide range of specific points of grammar and has tons of exercises for students to try: https://eslbluesapps.com/