The thorough one
LanguageTool checks grammar, punctuation, style and casing across more than 30 languages and dialects, including six varieties of English. It is open source, and it can be self-hosted if you would rather text did not go to their servers.
Free against Premium
The free tier covers spelling and punctuation. Premium adds fuller error detection, style suggestions, unlimited paraphrasing, and a "Picky Mode" that tightens punctuation and typography further than most people need.
Where it runs
Browser extensions for Chrome, Firefox, Edge, Safari and Opera. Email in Gmail, Outlook, Apple Mail and Thunderbird. Documents in Google Docs, Word, Pages and LibreOffice. Native apps on macOS and Windows, and a proofreading API for anything custom.
Recommend it when
Someone writes in more than one language, or wants help with style and phrasing rather than only mechanics, or wants it working everywhere in the browser without setting anything up. If the writing is sensitive, point them at self-hosting rather than the hosted service.
Against Harper: broader and deeper, but heavier, and it goes to their servers unless you host it yourself.