The main focus of this free website is grammar. You'll find a wide range of resources here including - lessons, PDF revisions sheets, practice exercises and videos. (check out the one where you need to find 10 grammar mistakes in an interview with Rafael Nadal)
The site can be used by students and teachers, and many materials can be used as homework. There is also a nice 'ASK US' section where the authors answer questions asked by email, twitter or facebook.