LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Grammarly - Clear, effective, mistake-free writing everywhere you type.
Gramlee.com - Online grammar check, proofreading and copy editing services.
GrammarChecker.net - Grammar Checker is an online spell check tool. It effectively works on English sentence correction, Punctuation mistakes of 28 other different languages.
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.
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.