After checking out various posts, these are the ones I'd like to recommend so far:
VOCABAHEAD - A word a day
LEVEL: Advanced Learners
There's a FREE version
Fun and intuitive way to learn hundreds of new words using humorous illustrations with audio. You have a list of words to learn from, with examples and drawings.
Watch words glide through your screen, some words may pause and show their definition, you can hear the word and add it to your favourites by tapping on +
Watch an overview
GENGO Language Audio Flashcards (also in English)
1. You learn the words/ sound with flashcards with images.
2. You test your knowledge by playing a game where you have to click on the image according to the word you hear.
I have this application in French and use it every day.
Really recommend it.
LEVEL: Beginner - Pre-Intermediate
LEARNING ENGLISH with the New York Times
Improve your reading, listening and vocabulary.
If you have already used Quizlet to make your own flashcards with words and definitions, you can use the following site to have access to your flashcard sets.