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

OpenTeacher - Free and open source tutor.

Grammarian PRO3 - Grammarian PRO3 is a feature-rich grammar checker that enables you to write better content without taking assistance from the English teacher.

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

GrammarChecker.net - Grammar Checker is an online spell check tool. It effectively works on English sentence correction, Punctuation mistakes of 28 other different languages.

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.