ProWritingAid is your free online writing editor and personal writing coach. Of course it checks your grammar but it does much more to help you improve your writing:
Online grammar and spelling checker;
Online plagiarism checker;
Improve readability;
Find overused words;
Improve dull paragraph structure;
Find repeated words and phrases;
Check for consistency of spelling, hyphenation, and capitalization;
Eliminate clichés and redundancies;
Create a word cloud of your text;
Eliminate vague, abstract, and complex words from your writing.