I just did a review of Grammarly, a grammar checker for English. In the comments on that post, for French, Damian recommended BonPatron (short texts are free, and the pro version is €9.99) and Antidote ($69).
Milena asked for suggestions in Spanish and Russian in another comment on that post. I've never made much use of grammar checkers, but, as I noted in my review of Grammarly, they could be quite useful. Accordingly, I'd like to broaden Milena's request: do you know of any other good grammar checkers in any language (including English)? If so, please drop a line in the comments!