Inkling by Tobias Oetiker

Inkling checks spelling, grammar and style in your web browser. It marks mistakes while you write in web mail, chat, social media, issue trackers, Google Docs and most rich text editors. Click a marked word to see the explanation and the suggestions; one more click applies a suggestion.

The checks come from a LanguageTool server that you set on the settings page. Run your own server (it is open source and runs on a laptop or a small server), use the one your team runs, or try the public server of LanguageTool. Inkling sends your text to that server and nowhere else. It works with every language that LanguageTool checks, among them English, German, French, Spanish, Dutch and Portuguese.

Inkling stays out of your way while you type: it waits until you pause, checks only the field you are typing in, and does not slow down long texts.

Features:
• Marks in three colors: red for spelling, amber for grammar and punctuation, blue for style
• Apply a suggestion with a click or a hotkey
• Your own dictionary, ignored suggestions and disabled rules
• Preferred variants such as de-CH or en-GB
• Switch checking on or off per site, or pause it in one tab
• Settings and popup in English and German

Inkling is free software under the Apache-2.0 license. It is based on the browser extension of Harper, the open source grammar checker by Automattic. It is not affiliated with LanguageTooler GmbH or with Automattic.

Source code: https://github.com/oetiker/inkling