Grammarly
Grammarly is an advanced freemium proofreading tool that scans written text and detects common grammar mistakes, including subject-verb agreement, article use, modifier placement, and more. Grammarly can even detect contextually erroneous words that are spelled correctly but used in the wrong place, such as lose/loose, affect/effect, and many other commonly conflated words. A basic edition of Grammarly is available for free, as a Chrome extension. Plagiarism checking and other advanced features are Premium only.