Macmillan Dictionary: A great online dictionary for learners. Includes a great feature where you can hear the pronunciation of word in both British and American English!
Word Reference: Very good website for checking the meaning of different words and the variety of ways that they can be translated
BBC Learning English: Many tips and resources to help you practice your English
Quizlet: How’s your vocabulary? Learn new words and practise ones you already know with this excellent website