www.rong-change.com is an excellent place for practice tests and exercises in listening, speaking, writing, grammar, vocabulary.
www.mes-english.com is a good place for resources like flashcards and language worksheets.
www.online-utility.org is another site that provides tips and exercises on reading, vocabulary, usage and writing.
Randall’s ESL Cyber Listening Lab—www.esl-lab.com— is a great resource for a large bank of listening and speaking samples of all kinds