Open Grammarly is a free, open-source alternative to premium grammar checking tools. This AI-powered grammar and spelling checker operates entirely within your browser, ensuring no subscriptions, data collection, or usage limits. It offers real-time grammar checking, smart corrections, and a familiar Grammarly-style UI with red underlines and clean correction popups. The tool functions seamlessly on any website with text inputs, textareas, and contenteditable elements. Users can choose from multiple writing modes, including Casual, Professional, and Academic styles, and adjust the aggressiveness of grammar checking. Privacy is a priority, with all API keys and settings stored locally in the browser. Open Grammarly allows users to bring their own AI models through OpenRouter, supporting models like GPT-4, Claude, and Gemini. The extension is built using modern technologies such as React, TypeScript, Tailwind CSS, and Vite, and is available as a Chrome extension. Installation is straightforward, with options to clone the repository and build the extension from source. The project is open source under the ISC License, inviting contributions from the community.