Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.

OpenTeacher - Free and open source tutor.

Grammarly - Clear, effective, mistake-free writing everywhere you type.

Vocabulary Miner - Flashcard app with offline learning, spaced repetition, no ads, export & import, statistics...

Slick Write - Slick Write checks your writing's grammar and style for free!

Anki - Anki is a program which makes remembering things easy. Because it's a lot more efficient than traditional study methods, you can either greatly decrease your time spent studying, or greatly increase the amount you learn.