Site Spell Checker adds a spelling panel to the block editor. Click Check spelling and it reviews the post title and content using an English (US) Hunspell dictionary, right in your browser.

Private. Your content is never sent to any server, including ours. There is no account, no API key, no analytics and no tracking.
Fewer false alarms. Contractions and possessives (don’t, it’s, O’Brien, Sarah’s) work with straight or curly apostrophes. Accented words (café, naïve) are accepted. Numbers, links, email addresses, code blocks and ALL-CAPS acronyms are skipped.
Names stay quiet. Capitalized words in the middle of a sentence are treated as names and skipped. You can turn this off.
Per-site dictionary. Add product names and jargon from the sidebar with Add to dictionary, or edit the list under Settings Site Spell Checker. Use Ignore to hide a word just for this editing session.
Lightweight. No admin notices, no banners, no redirects. Assets load only in the post editor, and the dictionary loads only when you run a check.

Need to check your whole site?

The plugin checks one post at a time. If you want every page checked at once, the separate paid service at sitespellchecker.com crawls your public site and sends you a report. The plugin only links to it (see Privacy for what the link passes). Every plugin feature works without it.

Privacy

The plugin makes no external HTTP requests, has no analytics or telemetry, and never sends your content anywhere. Spell checking runs in JavaScript in your browser. The only requests are for the plugin’s own dictionary files on your site and, when an administrator uses Add to dictionary, the WordPress REST API on your own site.

If you click the Scan every page of your site link (in the editor sidebar and on the settings page), your browser opens sitespellchecker.com with your site’s domain (for example domain=example.com) and attribution parameters (ref=wp-plugin, utm_source=wordpress, utm_medium=plugin, utm_campaign=editor-sidebar or settings) in the URL. Nothing is sent unless you click it, and no post content is included. sitespellchecker.com’s privacy policy applies to that visit.

Third-party libraries

Typo.js by Christopher Finke, Modified BSD License (GPL-compatible). Included unminified in assets/typo.js. License: assets/LICENSE-typo.txt.
en_US Hunspell dictionary from SCOWL by Kevin Atkinson and contributors, under a permissive MIT/BSD-style license (GPL-compatible). Files: assets/en_US.aff and assets/en_US.dic. License and credits: assets/README_en_US.txt.