Memrise is a website that offers courses based on a wide variety of topics. The section devoted to languages is really interesting as you will definitely find something you like. A lot of vocabulary courses are available with prepared lists of words in English only. The principle is to test you again and again, in a fun way,  to consolidate  your knowledge of vocabulary. You can then easily see and follow your progress.

Quizlet is a free website providing learning tools for students, including flashcards, study and game modes. You can create for your own study sets of vocabulary with definitions.