BonPatron is a free spelling and grammar checking site for French learners, but it is also suitable for native French speakers.
BonPatron’s developers target errors made by FSL learners — when the site detects an error, you will receive an explanation for how to improve it. You are encouraged to use the site at each stage of the writing process because when you correct one error, the program may identify new ones. The three categories of errors fall under misspelled words, structures to be verified, and structures to be corrected. The rules for error identification are created by patterns found in user submissions — so, if you are the first user to ever make a specific error, BonPatron may not be able to identify it unless it is based on a grammar rule that usually causes problems for English speakers. Feel free to make suggestions so that the developers can improve the program!
The free version allows you to check 3000 characters at a time. There is also a premium version that offers additional features, such as a verb conjugator, a summary of errors, and interactive grammar exercises for your errors.
Compared with Reverso, BonPatron does a better job of correcting general mistakes, such as capitalization or inappropriate commas. It also does a better job of catching errors related to inappropriate accordance of genders or numbers from “The House of Être” verbs.