LanguageTool

LanguageTool’s multilingual grammar, style, and spell checker is used by millions of people around the world. Receive tips on how to improve your text while typing an e-mail, a blog post or just a simple tweet. Whatever language you're using, LanguageTool will automatically detect it and provide suggestions. To respect your privacy, no text is stored by the browser add-on. LanguageTool is an open-source proofreading software for English, French, German, Polish, and more than 20 other languages.

Its goal is to be a major open-source tool in the field of computer assisted writing. It helps users with grammar checking, writing style etc. and can be integrated with productivity tools like Word, LibreOffice and OpenOffice as well as browsers like Chrome and Firefox