I have been using Quizlet for a long time and always recommend it to my students who occasionally have trouble remembering new words and expressions. With Quizlet you can quickly build up your vocabulary by creating your own module with new words and practicing them with cards, games and exercises.
I use British Council to prepare for classes as inspiration and to make presentations. A great choice for those who study and teach English.
A great resource for broadening your horizons and increasing your vocabulary. There are many videos on the TED site, after which you can do assignments, discuss the topic in chat with other people who are learning English.
Informative site, where the theory is clearly explained, and you can find many useful exercises to practice the new grammar material or consolidate the old.
A handy dictionary for both translators and students, where you can find many meanings of a single word. I use it both in my English classes and for translations.
On this site, I use videos with exercises to reinforce grammatical tenses, which I give as homework. When exercises get boring, this site will help make learning English a fun process.