BonPatron is a free French grammar corrector. Most professors allow and even encourage its use, but when in doubt, make sure to ask if they allow the use of this resource. The trick with BonPatron is to run your text through the program, look up and fix the errors that the program points out, and then run you text through the program again- repeat until you are satisfied with your text. Another thing to note is that BonPatron doesn’t correct errors; it only points them out without considering context. A monolingual dictionary is a great tool to use alongside BonPatron to look up contextual errors, verb constructions, etc. Finally, BonPatron doesn’t catch everything! Make sure to always edit your texts manually.
Guide de Grammaire (bonpatron.com/guide/1/)- BonPatron also has a great grammar guide where you can look up the errors that the program points out in your text. Le Guide de Grammaire is a simple and detailed grammar reference. It has everything you need on one page: verbs, adjectives, pronouns, prepositions, conjunctions, Anglicisms, etc.