English Club has lots of information, quizzes and guides to help you understand and practice English better. Their forums are a great place to chat with other learners about things that interest you.
The BBC is possibly one of the richest sources of high-quality, free English learning material on the Internet. Their Learning English site has so many sections I really can’t talk about all of them here (although I will say Words in the News is a very good place to start). Skillswise has excellent sections on spelling, reading, and writing. Their Video Nation site has interviews with real people and a short quiz afterwards to see if you...
Raymond Murphy’s Grammar in Use Intermediate is one of the most comprehensive books around of grammar rules and exercises. Every student should have this. There is also Advanced Grammar in Use, and while it is very complete I think it is appropriate for an extremely high level (Cambridge Proficiency Exam). A good exercise book in between these two is Cambridge Grammar for CAE and Proficiency.
English Page is a good collection of fundamental grammar rules with explanations, examples and exercises. A good first reference point.