Is this really free, with no signup?

Yes. The grammar engine is an Apache-2.0 licensed dictionary that runs in your browser, so there is no per-word server cost to recoup and nothing to sign up for.

Does my text get sent to a server?

No. The dictionary is downloaded to your browser once and every check runs locally after that — including offline. Confidential drafts, legal text and medical notes never leave the machine.

Which English does it check?

American, British, Canadian, Australian or Indian — switch the dialect and the spelling rules change with it.

It flagged a name or a term I use. Can I teach it?

Yes. Add the word to your personal dictionary, or ignore that specific complaint. Both are stored on this device only.

What is checked besides spelling and grammar?

Style: clichés, filler words, hedging, weak verbs, passive voice and repeated words. Those checks are plain rules and work even where the dictionary isn't available.

Free Online Proofreader — Grammar, Spelling and Style

Proofread a draft for spelling, grammar, punctuation, clichés, filler and passive voice — all in your browser, with every suggestion shown before it is applied.