LanguageTool is a free and open source multilingual spelling, style, and grammar checker that helps correct or paraphrase texts - eviltoast
LanguageTool - Online Grammar, Style & Spell Checker
languagetool.org
LanguageTool is a free grammar checker and paraphraser for English, Spanish, and 30 other languages. Instantly check your text for grammar and style mistakes.

Free and open source version works as Chrome browser extension or with Word