25 Common Grammar Mistakes - resource to use with intermediate and advanced students - article has dozens of curated links to grammar resources and practice exercises
Grammarly - a tool for improving writing, especially for pointing out obvious errors. The free version makes suggestions about spelling, grammar, and punctuation and gives short explanations via sidebar commentary. The free Grammarly desktop app lets you export in MS Word docx format. Android and iPhone apps are available in the App stores.