I describe Grammarly as Spell Check on steroids. It gives suggestions on sentence structure, spelling grammar, and much more. It can natively integrate with major web browsers (to check web-based writing) and Microsoft Office. It also has a native Mac app. They also just recently launched an iOS keyboard.
I put Grammarly in the I wish this had been around when I was in college bucket. While you can access it in multiple ways, I typically just drop first drafts of whatever I am writing into the web interface. I find it very helpful to work through its suggestions before submitting articles.
Grammarly is free to use, and there multiple plans to unlock the premium version.