ESL Lounge: A comprehensive website that offers test-prep, listening and reading practice, as well as some grammar exercises. A good place to start is the advanced level exercises, as the intermediate are fairly simple.
Vocabulary Exercises for the Academic Word List: Improve your vocabulary by learning some more words from the Academic Word List. This website separates the list into 10 easy to digest subsets.
Intermediate and Advanced Level
English Level Test: On this site, you can take 4 different level quizzes to get some immediate feedback on your overall language level. There are quizzes in Listening, Reading, Vocabulary, and Grammar.
Idioms & Axioms currently used in America: Add some idioms to your vocabulary! The idioms are defined as well as classified as to whether they are common, polite, or vulgar.
Study Zone: Choose your level (Upper Intermediate or Advanced recommended) and you will have access to reading, vocabulary, and some grammar exercise.