LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Grammarly - Clear, effective, mistake-free writing everywhere you type.
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.
Gramlee.com - Online grammar check, proofreading and copy editing services.
Stylewriter - StyleWriter edits any document.
Wordvice AI - Wordvice AI is an online writing assistant powered by GPT models. It features a suite of revision tools, including the AI Proofreader, AI Paraphraser, AI Summarizer, and AI Translator. Free and Premium plans are available.