Synonymful - Improve your vocabulary and communicate like a pro.

Reji - Improve your vocabulary hassle-free 📚

Duolingo - Duolingo is a free language learning app for iOS, Windows and Android devices. The app makes learning a new language fun by breaking learning into small lessons where you can earn points and move up through the levels. Read more about Duolingo.

Vocly - Learn vocabulary in 50 languages with fun games

Vocab Swap - Learn a language while you browse the internet

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.