Grammarly

I had my students from a writing co-op use this to edit their papers before submitting them to me. I was surprised at how many grammar errors it picked up on. It helped me greatly as I did not have the time to heavily edit all my student's papers or have the confidence to catch everything.
The things I liked the most was when errors were caught, a thorough explanation was given for the suggested correction. Students had the choice as to whether they wanted their work changed or not and they learned more about grammar and language usage along-the-way.
It is expensive, however, they often have a "back-to-school" sale each year which I will take advantage of again if it is offered.