It's a very useful site both fir teachers and students. If you're a student you can improve your grammar and build up vocabulary. Teachers can find lesson plans and different material to be used in classes. I find especially useful material on Reading Comprehension skills (http://esl.about.com/od/englishreadingskills/English_Reading_Comprehension_Skills_for_ESL_EFL_Learners.htm)

A great collection of printable worksheets for students of different age groups and with different levels of English. Registration is needed to use the resources (it's free of charge)