Harper: Free, Open Source Grammar Checker

Harper (writewithharper.com) is a grammar checker that runs entirely on your own machine — no account, no cloud, no LLM, nothing you write ever leaving your computer.

It's open source, free with no paid tier waiting behind it. Extensions for Chrome and Firefox, a Mac app, plugins for VS Code, Obsidian, Neovim and Zed if that's where you work.