Today I will comment on one of this comprehensive sites you can go to whenever you look for something new to learn. Usingenglish.com was opened in 2002 and is written by a group of teachers. The reference section includes a good glossary of idioms, phrasal verbs, irregular verbs and grammar terms.
Students includes tests and polls and articles on English usage. At the English ESL forum, they answer your questions about English usage.
Testing is a good source for English tests and quizzes, questions bank and selected online tests.
Teachers is a good section with articles on teaching uses and resources written by other ESL teachers.