Comment by dspillett
You can also run your own local instance for the in-browser checking, which is handy for me as I need to be careful about sending text off to another company in another country (due to both client security requirements and personal paranoia!).
You don't get the AI based extras like paraphrasing, and the other bits listed in as premium only (https://languagetool.org/premium_new), but if you install the n-gram DB for your language (https://languagetool.org/download/ngram-data/) I found it at least as good as, for some examples better than, Grammarly's free offering last time I did a comparison.
Replying to self as I'm too late to edit: I left the wrong link for ngram info, the download location instead of the instructions for use which are at https://dev.languagetool.org/finding-errors-using-n-gram-dat...