Learn the words you need for your subject tests, and many useful general words too!
Match the words and pictures - for beginners!
The Internet Picture Dictionary - basic vocabulary with various quizzes
Basic English picture wordlist from Wikipedia. Each picture links to information about that word.
These pages will teach you some new idioms - this is one of the hardest aspects of learning English!
Phrasal verbs are a most important aspect of English vocabulary - learn some new ones in these pages.
A list of the words that even many native speakers get mixed up.
Lots of interesting stuff about vocabulary, including various online tests.

Academic Vocabulary
Simple English Wikipedia's version of the Academic Word List by Averil Coxhead. Highly recommended for ESL students.
Gap filling exercises based on the Academic Word List.
Exercises from the Academic English Cafe.
Exercises from the ESLGold website.
A complete list of English prefixes and suffixes, with their meanings.